New 15/16 master cylinder in a 74 620

[Richie]

74 620, I have disc in the front drums in the rear. I am looking to install a 15/16 brake master cylinder from a 280. Would i need a portioning valve adjuster if i install this master cylinder?

 

I am installing this cylinder because i have waaay to much travel in my brake pedal and it is really hard to get the brakes to lock up correctly. Thank you. 

[mklotz70]

Short answer......it would be best.

 

According to what I've found on Datsun/Nissan m/c's, the biasing(proportioning) is built into the m/c. I don't really trust that, but I understand why they say that.  I think the m/c controls the "timing" of the fluid application, but I don't see it affecting the pressure.  The push rod pushes the first piston(on the 15/16" m/c, that's the front circuit) which pushes a spring, which pushes the second piston(rear circuit) which then pushes against a spring.  The difference between the springs creates the biasing.  The rear circuit spring should be pretty light....you want the rears to engage first to help keep the nose dive to a minimum.  Once that pressure starts to build up as the shoes engage the drums, the rear piston won't move as easily and the first piston will start applying pressure to the front brakes.  Calipers don't require much movement to engage.  With that pressure built up, I'm pretty sure the fluid pressure to the front and rear should be pretty similar.  How much work(braking) is being done differs because the fronts have a small piston acting on large pistons....multiplying the force, while the opposite happens in the rear.  The rears have way more contact surface on the drums and a lot less weight/traction.  

 

I should redo these vids, but I think you can see the piston and springs and their arrangement.  You may already know this stuff, but with the question you're asking, maybe not.

 

I used that 15/16" m/c on my wife's 521, but I used the combo valve from an S10 pickup.  It had front disc and rear drums and the weight bias front to rear would be more similar than that of a 280.  A combo valve has a biasing switch(safety in case one circuit loses fluid), a proportioning valve and a hold off valve.  Basically, the hold off valve holds off the front brakes until the rear have had a chance to engage. Now that I'm thinking about it, the hold off valve may be redundant, but I installed it when I built the first set of disc brakes for it.  I've never driven the truck without the combo valve after the conversion.  I've never recommended for anyone to use the combo valve, only the adjust proportioning valve. The plumbing is way easier!! lol

 

I used an adjustable prop valve in my NL320.  I had the knob poke up through a hole in the floor under the driver's seat.  It rains a lot around here and I wanted to be able to get to it if it was raining.....without getting out of the truck.  I'm so sweet, I'd melt instantly.....ask anyone here that knows me. 😉  lol

 

There's a good chance that the rear shoes are not adjusted close enough.  Excess travel on drum brakes makes for a lot of extra pedal travel.  Don't adjust them so there's no rubbing.  I find that a bit of scuffing means you're close without dragging.  The scuffing will go away pretty quickly when you start driving it.  

 

Which brakes do you have up front?  If you only want to take the truck down and work on the brakes once, put the prop valve in when you change the m/c.  Otherwise, put the m/c in and drive it.  On dry roads you probably won't be able to set it very easily.  If you have a dirt road near you, test it there.  If the rears lock up too quick and easy on that road, then you'll know that they'll lock up early in the rain too.  Your fronts are going to stop you whether your rears work or not.  If your rears lock up first on dry pavement, you're definitely going to want to put the prop valve in.

Something else to consider......if you just replaced your pads and shoes, you need to wait until you properly "bed" them before attempting panic stops or trying to adjust the biasing.  Brand new pads are pretty spongy compared to pads that have some time on them.

 

Gotta run...way past my time limit. lol  I hope some of that helped.

[Richie]

2 hours ago, mklotz70 said:

Short answer......it would be best.

 

According to what I've found on Datsun/Nissan m/c's, the biasing(proportioning) is built into the m/c. I don't really trust that, but I understand why they say that.  I think the m/c controls the "timing" of the fluid application, but I don't see it affecting the pressure.  The push rod pushes the first piston(on the 15/16" m/c, that's the front circuit) which pushes a spring, which pushes the second piston(rear circuit) which then pushes against a spring.  The difference between the springs creates the biasing.  The rear circuit spring should be pretty light....you want the rears to engage first to help keep the nose dive to a minimum.  Once that pressure starts to build up as the shoes engage the drums, the rear piston won't move as easily and the first piston will start applying pressure to the front brakes.  Calipers don't require much movement to engage.  With that pressure built up, I'm pretty sure the fluid pressure to the front and rear should be pretty similar.  How much work(braking) is being done differs because the fronts have a small piston acting on large pistons....multiplying the force, while the opposite happens in the rear.  The rears have way more contact surface on the drums and a lot less weight/traction.  

 

I should redo these vids, but I think you can see the piston and springs and their arrangement.  You may already know this stuff, but with the question you're asking, maybe not.

 

I used that 15/16" m/c on my wife's 521, but I used the combo valve from an S10 pickup.  It had front disc and rear drums and the weight bias front to rear would be more similar than that of a 280.  A combo valve has a biasing switch(safety in case one circuit loses fluid), a proportioning valve and a hold off valve.  Basically, the hold off valve holds off the front brakes until the rear have had a chance to engage. Now that I'm thinking about it, the hold off valve may be redundant, but I installed it when I built the first set of disc brakes for it.  I've never driven the truck without the combo valve after the conversion.  I've never recommended for anyone to use the combo valve, only the adjust proportioning valve. The plumbing is way easier!! lol

 

I used an adjustable prop valve in my NL320.  I had the knob poke up through a hole in the floor under the driver's seat.  It rains a lot around here and I wanted to be able to get to it if it was raining.....without getting out of the truck.  I'm so sweet, I'd melt instantly.....ask anyone here that knows me. 😉  lol

 

There's a good chance that the rear shoes are not adjusted close enough.  Excess travel on drum brakes makes for a lot of extra pedal travel.  Don't adjust them so there's no rubbing.  I find that a bit of scuffing means you're close without dragging.  The scuffing will go away pretty quickly when you start driving it.  

 

Which brakes do you have up front?  If you only want to take the truck down and work on the brakes once, put the prop valve in when you change the m/c.  Otherwise, put the m/c in and drive it.  On dry roads you probably won't be able to set it very easily.  If you have a dirt road near you, test it there.  If the rears lock up too quick and easy on that road, then you'll know that they'll lock up early in the rain too.  Your fronts are going to stop you whether your rears work or not.  If your rears lock up first on dry pavement, you're definitely going to want to put the prop valve in.

Something else to consider......if you just replaced your pads and shoes, you need to wait until you properly "bed" them before attempting panic stops or trying to adjust the biasing.  Brand new pads are pretty spongy compared to pads that have some time on them.

 

Gotta run...way past my time limit. lol  I hope some of that helped.

wow, thank you. 

So I have the D21 disc conversion from silvermine. And the rear drums do need to be adjusted because i have to pull the ebrake all the way up for it to engage and even still you can push the car while the ebrake is on. The only problem i have when adjusting is the adjuster on the driver side rear is not adjusting anything. I got it to loosen up after putting a torch to the base for a little, however i get like 1 maybe 2 clicks and it stops. 

[mklotz70]

pull the adjuster apart and grease it. It will save you a ton of grief adjusting them later.

[datzenmike]

Mike, I thought the rear wheel cylinder diameter was primarily responsible for bias. The system was engineered, (probably trial and error) if the rear locks up too soon then go to a smaller wheel cylinder. 

 

As to the dual 15/16" MC, the rear piston moves first and builds pressure in the front circuit, it has to and it should, as the fronts do almost all the work stopping. Although there is a spring between the rear and front piston it would have to be enormously stiff to transmit motion from the back to the front. At some point the rear piston physically contacts the front piston and pushes it ahead. I think the very front spring and the one between the front and rear pistons are more about pushing these pistons back to their return positions. Just my opinion.

[Richie]

 

2 hours ago, mklotz70 said:

pull the adjuster apart and grease it. It will save you a ton of grief adjusting them later.

I have two new adjusters, I just hate working on drums....

I cant get the rears to lock up at all. I know the fronts do most of the stopping power but should the rears lock up first then the fronts?

[mklotz70]

I think you're right Mike.....as I said, I don't think the m/c does much proportioning of the pressure like they say, but more so the timing of the pressure application.  If the springs were only there to return the pistons to their resting place, they would be the same, but they're not.  When I refer to rear and front, it's the circuit, not necessarily the piston position.  the front circuit(piston closest the pedal or the rear one in the m/c) has to travel farther without building up much pressure so that enough fluid gets moved in the rear circuit to get the pads to contact.  Once both the rears and the fronts are in contact, then the fluid pressure and piston sizes can come into play.  Smaller rear slave cylinders would help shift the bias to the front brakes, but it's easier to put in a proportioning valve to help cut the pressure to the rear brakes.  At least that's my understanding.  

 

Richie.....the m/c only has so much travel in it.  If it (the push rod) runs out of travel before the rear slave cylinders can push the shoes into full contact, the rears will never lock up.  If you get good feeling brakes when you pump the pedal once, I'd go straight to the rear brakes and adjust them.  A bigger m/c might help over come that lack of volume to the rear cylinders, but it won't fix the problem of them not being adjusted correctly.  You're just going to have to suck it up and deal with the rear brakes.  Almost no one cares for drum brakes.....which is the #1 reason they change to disc brakes.  The ability to stop is not a significant reason for upgrading the rears.  Drums have way more contact area and require less leg effort.  

 

The rears lock up first because there's a lot more brake and a lot less weight.  The additional braking power in the rear is because the trucks were designed to carry loads.  For daily driving with no load, it's not beneficial for the rears to lock up first.....hence the reason a proportioning valve is installed.  You can cut back on the pressure to the rears for daily driving and then increase it when you are carrying heavy stuff.  It's always dry down there so locking up the rears takes a lot. Up here, in the rain, having the rears lock up too easily on a wet road is a good way to lose your truck.  If you have to panic stop in traffic and you're not going perfectly straight ahead, the rear can come around quicker than you can react.

[Richie]

On 8/24/2022 at 9:41 AM, mklotz70 said:

I think you're right Mike.....as I said, I don't think the m/c does much proportioning of the pressure like they say, but more so the timing of the pressure application.  If the springs were only there to return the pistons to their resting place, they would be the same, but they're not.  When I refer to rear and front, it's the circuit, not necessarily the piston position.  the front circuit(piston closest the pedal or the rear one in the m/c) has to travel farther without building up much pressure so that enough fluid gets moved in the rear circuit to get the pads to contact.  Once both the rears and the fronts are in contact, then the fluid pressure and piston sizes can come into play.  Smaller rear slave cylinders would help shift the bias to the front brakes, but it's easier to put in a proportioning valve to help cut the pressure to the rear brakes.  At least that's my understanding.  

 

Richie.....the m/c only has so much travel in it.  If it (the push rod) runs out of travel before the rear slave cylinders can push the shoes into full contact, the rears will never lock up.  If you get good feeling brakes when you pump the pedal once, I'd go straight to the rear brakes and adjust them.  A bigger m/c might help over come that lack of volume to the rear cylinders, but it won't fix the problem of them not being adjusted correctly.  You're just going to have to suck it up and deal with the rear brakes.  Almost no one cares for drum brakes.....which is the #1 reason they change to disc brakes.  The ability to stop is not a significant reason for upgrading the rears.  Drums have way more contact area and require less leg effort.  

 

The rears lock up first because there's a lot more brake and a lot less weight.  The additional braking power in the rear is because the trucks were designed to carry loads.  For daily driving with no load, it's not beneficial for the rears to lock up first.....hence the reason a proportioning valve is installed.  You can cut back on the pressure to the rears for daily driving and then increase it when you are carrying heavy stuff.  It's always dry down there so locking up the rears takes a lot. Up here, in the rain, having the rears lock up too easily on a wet road is a good way to lose your truck.  If you have to panic stop in traffic and you're not going perfectly straight ahead, the rear can come around quicker than you can react.

Its not that i hate drums, I know they cover more surface than the disc. To be honest I am just trying to achieve less travel in the pedal, and e brake actually working. I did change everything in the drums besides the hardware because I couldnt find the spring set. But since i changed the rear drums the ebrake is lacking. So it is my fault. I will suck it up and adjust them. I just hope this MC and adjusting will help me achieve less travel in the pedal, and ebrake working properly.

[Stoffregen Motorsports]

Late to the party here.

 

Is the master you are using from a car with rear disc brakes? If so, you should install a residual valve in the rear line. I would bet money that this would fix the problem.

[datzenmike]

Yes to the above. My 15/16 is from a rear disc 280zx. I took the residual valve from a drum brake master and swapped it into the zx master. Rear drum valves retain a 10-12 PSI pressure in the lines, a disc brake valve is 2? psi. Just enough to keep the pads and rotor clean but not drag.

 

 

As the rears do much less braking than the fronts due to weight transfer, they are more than adequate and disc's won't stop you any faster. Their main advantage is almost zero maintenance (adjustments for wear) and replacing the pads is a 5 min. job. Yes they are much more fade resistant but when was the last time you ever used your brakes to the point of boiling the brake fluid? Yeah, never. 

[Richie]

1 hour ago, datzenmike said:

Yes to the above. My 15/16 is from a rear disc 280zx. I took the residual valve from a drum brake master and swapped it into the zx master. Rear drum valves retain a 10-12 PSI pressure in the lines, a disc brake valve is 2? psi. Just enough to keep the pads and rotor clean but not drag.

 

 

As the rears do much less braking than the fronts due to weight transfer, they are more than adequate and disc's won't stop you any faster. Their main advantage is almost zero maintenance (adjustments for wear) and replacing the pads is a 5 min. job. Yes they are much more fade resistant but when was the last time you ever used your brakes to the point of boiling the brake fluid? Yeah, never. 

 

1 hour ago, Stoffregen Motorsports said:

Late to the party here.

 

Is the master you are using from a car with rear disc brakes? If so, you should install a residual valve in the rear line. I would bet money that this would fix the problem.

yea Mike youre right. Basically i just want it to be safe to drive at top speed 50 and stop correctly without any issues. Safety is my concern also. I have to press the pedal all the way down just for it to stop. But i have gotten the fronts to lock up which is good. 

 

Ok soooo i can put the 15/16 MC on with the 3/4 rear residual valve from my MC right now. Adjust the rear breaks and good. Thank you.

[datzenmike]

 

The residual valves are right where the brake lines leave the master. Remember that the front most circuit on the master cylinder is for the rear brakes. You want to swap the front most residual valve. I could not see any difference between a drum and a disc residual valve so don't get them mixed up!!!!!

 

 

[Richie]

8 minutes ago, datzenmike said:

 

The residual valves are right where the brake lines leave the master. Remember that the front most circuit on the master cylinder is for the rear brakes. You want to swap the front most residual valve. I could not see any difference between a drum and a disc residual valve so don't get them mixed up!!!!!

 

 

I previously did this like you told me before. I put a 75 MC residual valve from the front disc so the psi wouldnt lock the disc pads. Now i am getting a bigger MC and will do the dame thing but for the rears. Thank you. 

[620slodat]

Just to be aware of this, for those people that deal with a lot of rain, or the once a year flood, the disc brakes are better because they deal better with moisture. The only reason is if you go through water the drum brakes be useless (no braking possible) for a little while, unless you know how to keep drum brakes dryer. Disc brakes will not usually be affected because of going through water. I have to go now, but there is a lot to be said about drums/discs and water.

 

Don

[datzenmike]

I think they might clear faster than drum but if submerged they stop working. I just hold the brake on with the left foot and step on the gas and the pads squeegee the water off the rotor. It's a bit scary if you are already braking but otherwise 5-6 vehicle lengths and they come back.

[mklotz70]

Richie.....you probably already know, but I haven't seen it mentioned......once you get the rear drums adjusted and have a good pedal feel...if the e-brake is still a long pull and not as effective as you want, there's an adjustment under the truck, typically where the single cable meets the rear cables.  You can tighten that up to get the e-brake to engage earlier.  You can engage the e-brake when you bleed the brakes.  Obviously, it has to be fully released when you adjust the shoes. 

 

Once you get the adjusters "rebuilt", adjust the shoes until the drag a bit firmly....then press the brake pedal a few times to get them to re-center.  Typically, after pressing the pedal, they won't drag....adjust again until they do.....apply the pedal a few more times.  Repeat until the drag feels the same before and after the pedal application.  Most drums won't be perfectly round and you'll have drag in a spot or two while rotating them.  You'll want a bit of drag at those spots, but not enough to make it that hard to rotate them.  I prefer to do it with the wheel/tire on.  The extra mass helps me get a better feel for just how much drag there is. I want to hear it scuff, but not drastically slow the rotation down. If doing it without the wheel/tire, it's a lot harder to gauge...almost any drag will make it stop with just your hand force.  Also make sure you have lug nuts holding the drum on if there's no wheel/tire.  If the scuffing/dragging is in certain places of the rotation, it can be a firmer drag.  If your drums are really true, and scuff all or most of the way around, you'll want it to be very light scuffing....at that point, back off the adjuster one click at a time until there's basically no scuffing.....this is only if your drums scuff through the entire rotation. Constant scuffing can create too much heat.  If they only scuff in one or two places, that will wear down pretty quickly and shouldn't cause a heat issue.  I think the manual says to back off the adjuster "X" number of clicks after you adjust it tight enough to stop the rotation.  You can start with that, but I would then tweak it from there.

 

Discs can sling the water off while drums fill up and have a harder time draining.  I've seen drilled drums before, but in SoCal, you wouldn't need it enough to do that.  I wouldn't do it even up here.  I think it weakens the drums too much to be good in the long run.  I'm not even a fan of drilled and/or slotted rotors up front on daily drivers.  For the pads to work their best, they need some heat and d/s rotors don't allow enough heat build up...typically.....on daily drivers.  

 

I agree that the biggest reason to go with rear discs is less maintenance.

 

On the stock residual valves, I've never seen a difference between the parts either, but it would have to be different spring rates for them to hold different pressures.  If it seems the stock versions don't work, you can get external residual valves in 2 or 10psi ratings.  They're really common in old hot rod builds where the m/c is under the floor board.

 

[datzenmike]

The FSMs say adjust till heavy drag then loosen 12 clicks. This seems an insane amount of looseness and I ignore and do like you do.

 

As to water, my 521 was full drum bakes and when off road, I often submerged them and the brakes were gone, even quite a while later they were weak so I drove with left foot on the pedal to warm and dry them out. My 620, and now my car, were front disc and they would fade, even the car but only if driving fast through substantial puddles at speed. They would clear much faster. Again left foot on brake for a few seconds did the trick. Even the heaviest rain has no effect. You have to drive through standing water to get this loss of function.

[Stoffregen Motorsports]

Wilwood makes inline residual valves. Here's a screenshot.

[Stoffregen Motorsports]

On the topic of rear drums vs rear discs - even GT spec 510 race cars were using finned aluminum rear drums from a 240Z well into the '90s. So while rear discs may work better at 10/10ths, the rest of the time, drums are just fine. I think a 510 with finned aluminum drums in the rear looks cooler anyway.

 

I do know that up until a couple years ago, 240z rear drums were available in the afermarket. And I have also heard that Triumph finned drums can be made to fit by drilling the stud holes - https://mossmotors.com/alfin-style-9-brake-drum-rear

[iceman510]

10 hours ago, Stoffregen Motorsports said:

And I have also heard that Triumph finned drums can be made to fit by drilling the stud holes - https://mossmotors.com/alfin-style-9-brake-drum-rear

 

"Come at a very reasonable price"?  My definition of reasonable is not $300/drum.  🤣  They do look much like the Datsun version though.

[Stoffregen Motorsports]

Pretty standard response from a Ratsuner.

 

Cheap bastards...

 

But yes, I would have expected the price to be more like $200 each, not $300 each.

[yenpit]

On 8/27/2022 at 10:07 AM, Stoffregen Motorsports said:

On the topic of rear drums vs rear discs - even GT spec 510 race cars were using finned aluminum rear drums from a 240Z well into the '90s. So while rear discs may work better at 10/10ths, the rest of the time, drums are just fine. I think a 510 with finned aluminum drums in the rear looks cooler anyway.

 

I do know that up until a couple years ago, 240z rear drums were available in the afermarket. And I have also heard that Triumph finned drums can be made to fit by drilling the stud holes - https://mossmotors.com/alfin-style-9-brake-drum-rear

 

Last I heard, the finned aluminum were NLA NLS, even in the aftermarket.  You can still buy NEW, but they are standard steel.  If anybody needs any finned aluminum ones, I have a stack of USED ones.  I have them machined & measured before I sell them, then they only need cleaning & sandblasting!

[Stoffregen Motorsports]

3 hours ago, yenpit said:

 

Last I heard, the finned aluminum were NLA NLS, even in the aftermarket.  You can still buy NEW, but they are standard steel.  If anybody needs any finned aluminum ones, I have a stack of USED ones.  I have them machined & measured before I sell them, then they only need cleaning & sandblasting!

This is a good resource. Though I'd hate to see all of them end up on cheap SOB's street cars when they deserve to be on vintage race cars.

 

Have you taken a look at the Moss Triumph drums? They look nearly identical and guys say they will work, though I've never actually tried them.

[iceman510]

The 73 I am assembling came with two sets of finned drums, but they were brand new Centric and not aluminum.  I sold them.  I don't understand why Centric offers a standard and a premium.  Looks like they are both iron.

 

I have at least a couple sets of the original AL ones around.  I will be putting one set on the street car (although Stoffregen might call me more names 😉).   For the record, I am NOT a cheap SOB in the Ratsun (read: datzenmike) tradition.  No muffler clamps on struts here.  🤣

 

Motorsport Auto says they are trying to get them made:  https://www.thezstore.com/page/TZS/PROD/24-5050A

 

 

There was a 280 in the j/yard here last year that still had them on it.  I should go back and see if it is still there...

Edited August 29, 2022 by iceman510

[Stoffregen Motorsports]

13 minutes ago, iceman510 said:

The 73 I am assembling came with two sets of finned drums, but they were brand new Centric and not aluminum.  I sold them.  I don't understand why Centric offers a standard and a premium.  Looks like they are both iron.

 

I have at least a couple sets of the original AL ones around.  I will be putting one set on the street car (although Stoffregen might call me more names 😉).   For the record, I am NOT a cheap SOB in the Ratsun (read: datzenmike) tradition.  No muffler clamps on struts here.  🤣

 

Motorsport Auto says they are trying to get them made:  https://www.thezstore.com/page/TZS/PROD/24-5050A

 

 

There was a 280 in the j/yard here last year that still had them on it.  I should go back and see if it is still there...

Yeah, what's with the whole cast iron finned drum thing? I get it, but don't get it.

 

Ok, not cheap bastard then. I'm glad you referenced Mike's suggestion on the strut mod. That had me cringing too.