Disc brake m/c conversion question

[Eastcoast521]

I am considering a front disk swap so I went and bought a book about brake systems and one interesting thing I learned, which seems counter-intuitive, is that to increase the master cylinder's output PSI you actually have to decrease the bore size. The output formula is pedal force divided by master cylinder's cross-sectional area. Also, a smaller bore results in more pedal travel compared to a bigger bore. According to the book, the % increase in bore size results in the same % decrease in pedal travel.

[Hoosier77]

what book is that you have? can i get it on amazon.com?

[Eastcoast521]

It's called "High Performance Brake Systems - Design, Selection and Installation" by James Walker. I got mine at Barnes and Noble. It's a very interesting book.

[ggzilla]

Yep, like Mike says (and the book says)

 

* Larger bore Master Cylinder reduces pedal travel, but increases the force you need to push with your leg

 

* Smaller bore Master cylinder reduces the force you need to push with your leg, but increases pedal travel

 

or put another way: The larger bore has a lower pressure output (so then you must press harder with your leg!)

 

I like a pedal that doesn't move too much, but don't like brakes that are so hard to push down on that I'd be afraid to loan it out... so in that case would fit a small booster (B210/610) or medium size booster (620/B310). The booster won't make the brakes any more effective but make it easier to apply.

 

I have a disconnected booster in my truck -- with a late-model large-bore Master Cylinder -- and it takes too much pedal effort to do a panic stop in my opinion to be comfortable. The previous owner disconnected it due to the twin sidedraft carburetors (the manifold doesn't have a vacuum tap to connect up the booster).

[mklotz70]

.....so what happened to Aaron...the originator?

[hawaii_620]

So I want more output PSi to my front D21 calipers on my 620, and I have a 280zx mc + 620 brake booster. My pedal already reaches close to if not the bottom of the pedal travel. So could I try a different SMALLER master cylinder to gain output PSI and to compensate for the extra pedal travel, just adjust the push rods so that it doesn't bottom out?

[mklotz70]

Smaller will get you more pressure, but it will also take MORE pedal travel.

 

If you have really firm brakes when they're engaged, but too much travel, the most likely culprit is the back brakes not being adjusted close enough.

 

Pushrod is simply an adjustment related to the pedal itself, not for pedal travel. With the pedal in it's up position...not pressed at all, there should be a tiny amount of looseness or play in the pushrod. If it's tight, loosen it. If it's sloppy loose, tighten it up. The pedal should only move about a 1/4"....check the factory manual for the spec......before all the slop in the clevis and rod are taken up and starts pushing the piston in the m/c.

[Hoosier77]

if you dont want it to bottom out try a 1 inch bore, i assure you it wont bottom out :lol:

[haubstr]

I feel like I'm beating a dead horse because I've read all of the posts about the disk conversion and still don't know if I have the right master cylinder or not...

 

I have disks on the front and back and I have what I believe is the 280ZX MC along with the Wilwood prportioning valve pictured below...

 

I have the rear (small resevoir) hooked up through the proportioning valve and it is bled and is fine... the front (large resevoir) however I can't get more than a drop out at a time even during the bench bleeding... I understand it might be bad but how do I confirm?

 

Also what is the purpose of the longer rod for this master? I understand the MC is longer but I push on my brake pedal and it only moves about 1 - 1 1/2" and it is so firm I can't push it in anymore and my back brakes work fine... which is also the the furthest resevoir...

 

As someone else has said "school me"

 

 

 

 

Thanks in advance

 

Jamie

[mklotz70]

What year of 280zx? '78 was disc/drum, '79 was disc/disc.....I think....I'm always wrong about the years....yello620 always corrects me :)

 

How exactly are you bench bleeding it? Short lines looped back into the reservoirs? How far are you pushing the pistons in? There's two pistons in the m/c. With it mounted in a vise, short lines in place.....and the ends in the reservoirs need to be submerged or they will suck air back in. Take a dowel or screwdriver and push the pistons in and watch for bubbles in the reservoirs. Don't push or release too fast. 2-3 seconds each way. You need to push in quite a ways...both pistons need to bottom out. Since they are "stacked" pistons, if the front circuit(rear) piston has air, it will not push the second piston until if physically bottoms out against it. Once is is bled and has all fluid, the fluid pressure will push the second piston.

 

Since you'll have the caps off the reservoirs when you do this, if the front circuit(rear res) is bad, it may shoot fluid back up into the res as you push.

 

If you have the m/c in the truck, you'll need a second set of hands to help. Get a long screwdriver....remove the push rod from between the pedal assy and the m/c. One of you crack open the rear circuit bleeder valve....the other, steadily push the piston in with the screw driver. Fluid should come out. When the screwdriver stops, or fluid stops, close the bleeder valve, but do not release the screwdriver. While still pushing, crack open the other bleeder valve. If it's working, fluid will come out and the screwdriver will go down further. If there is neither fluid or movement from the screwdriver, close the valve and then slowly release the screwdriver. Repeat again, but only do the front circuit this time.....since we know the rear is working. It's okay to have the valve open while you're pushing...just make sure it's closed before the screwdriver is released. You may need to do this about 5 times? If you do not get any fluid from the front circuit, remove the bleeder valve and see if fluid comes out. If it does, something's wrong with the bleeder valve.

 

this is soooo much easier to explain in person than in print!! If you're not getting fluid out of the front circuit even with the bleeder out, there may be some crap in the hole in the bottom of the reservoir cup. You can push a piece of vacuum tubing onto the bleeder valve......with the valve open a bit, gently blow into the tube......if you have the cap off the reservoir, you should see bubbles. Wear safety googles or do not hover over the reservoir.....you don't want brake fluid getting in your eyes!!

 

 

Longer pushrod?? You have a 521? The pushrod on the stock m/c was captured and not the correct style to be used with the 280 m/c. If you got creative and used the 521 pushrod....which is doesn't look like since you have an adjustable on there......that would be your problem since the piston is not returning completely because of the pushrod.

[mklotz70]

metalmonkey has a similar thread....similar problem, but with the opposite circuit.......he exchanged the m/c on warranty and that fixed the problem. You might just consider doing that :)

[datzenmike]

Mike it was stated earlier that the B210/610 boosters were 'smaller' that the 620s. By 'smaller' do you think they apply less assist to the smaller masters that the 620?? What I want to know is... will a 620 booster provide more assist than a 610?

 

I'm attempting to replace my 710's 3/4" master with a 15/16 from a ZX. This will very likely increase the effort needed to work it. A more powerful booster would be nice. I can't install a longer pedestal to move the larger diameter booster forward to clear the clutch master/steering column because of the strut tower. A stronger booster would be ideal.

[burrito213]

You are right mklotz70, I read the hints and tips a few times before it registers in my head. Maybe there should be a video how too. Not to say you specifically, but to anyone who knows a skill or trick

[mklotz70]

Mike....the larger the diameter, the more assist. You could go with the 13/16" for a compromise :)

 

 

I've thought about doing a vid....I might...just don't know that I'll have time tomorrow.

[haubstr]

What year of 280zx? '78 was disc/drum, '79 was disc/disc.....I think....I'm always wrong about the years....yello620 always corrects me :)

 

How exactly are you bench bleeding it? Short lines looped back into the reservoirs? How far are you pushing the pistons in? There's two pistons in the m/c. With it mounted in a vise, short lines in place.....and the ends in the reservoirs need to be submerged or they will suck air back in. Take a dowel or screwdriver and push the pistons in and watch for bubbles in the reservoirs. Don't push or release too fast. 2-3 seconds each way. You need to push in quite a ways...both pistons need to bottom out. Since they are "stacked" pistons, if the front circuit(rear) piston has air, it will not push the second piston until if physically bottoms out against it. Once is is bled and has all fluid, the fluid pressure will push the second piston.

 

Since you'll have the caps off the reservoirs when you do this, if the front circuit(rear res) is bad, it may shoot fluid back up into the res as you push.

 

If you have the m/c in the truck, you'll need a second set of hands to help. Get a long screwdriver....remove the push rod from between the pedal assy and the m/c. One of you crack open the rear circuit bleeder valve....the other, steadily push the piston in with the screw driver. Fluid should come out. When the screwdriver stops, or fluid stops, close the bleeder valve, but do not release the screwdriver. While still pushing, crack open the other bleeder valve. If it's working, fluid will come out and the screwdriver will go down further. If there is neither fluid or movement from the screwdriver, close the valve and then slowly release the screwdriver. Repeat again, but only do the front circuit this time.....since we know the rear is working. It's okay to have the valve open while you're pushing...just make sure it's closed before the screwdriver is released. You may need to do this about 5 times? If you do not get any fluid from the front circuit, remove the bleeder valve and see if fluid comes out. If it does, something's wrong with the bleeder valve.

 

this is soooo much easier to explain in person than in print!! If you're not getting fluid out of the front circuit even with the bleeder out, there may be some crap in the hole in the bottom of the reservoir cup. You can push a piece of vacuum tubing onto the bleeder valve......with the valve open a bit, gently blow into the tube......if you have the cap off the reservoir, you should see bubbles. Wear safety googles or do not hover over the reservoir.....you don't want brake fluid getting in your eyes!!

 

 

Longer pushrod?? You have a 521? The pushrod on the stock m/c was captured and not the correct style to be used with the 280 m/c. If you got creative and used the 521 pushrod....which is doesn't look like since you have an adjustable on there......that would be your problem since the piston is not returning completely because of the pushrod.

 

 

So before I even try anything else with this MC let me ask you a question... Do I even have the right one to run in my 521? According to everything I read, the MC I have, which is the one for front Disk rear drum, will not run on my 4 wheel disk set up... Correct?

 

If I need a different one then which one should I get? I will not be running a booster and I need the smallest bore that will work for the 4 wheel disk...

 

Thanks in advance

 

Jamie

[mklotz70]

Will it work? The short answer is yes. Guys are out there running front disc, front disc/rear disc setups with the stock, single reservoir m/c. I don't recommend that, but they say it works. My understanding is that there's a slight difference in the amount of fluid moved by the m/c. More is needed for drum wheel cylinders. If the shoes are adjusted pretty closely, I doubt a difference would be noticed. If you have an adjustable proportioning valve installed, my guess would be you're just fine. I haven't done any work with rear discs yet, so I can't say from experience. I'm guessing that Beebani's customers that have bought/installed the rear disc kits would have a better opinion on which m/c to use. Have you pm'd Beebani?

[datzenmike]

Mike....the larger the diameter, the more assist. You could go with the 13/16" for a compromise :)

 

 

I've thought about doing a vid....I might...just don't know that I'll have time tomorrow.

 

 

 

My mistake. I have a 710 booster and plan to run a 15/16" master for my Maxima calipers. In the quote below it seems that the 620 booster would be stronger and reduce the effort. So is the 620 booster stronger than a 610/710 booster? I have a 620 booster and they look the same 'size' but does it doo more work?

 

 

 

 

I like a pedal that doesn't move too much, but don't like brakes that are so hard to push down on that I'd be afraid to loan it out... so in that case would fit a small booster (B210/610) or medium size booster (620/B310). The booster won't make the brakes any more effective but make it easier to apply.

 

 

[hawaii_620]

I'm looking to replace my remanufactured M/C. So I called the local Nissan dealer around here and a new M/C for a '78 280z is $410 special order. Ummm, I'll pass on that for now.

 

What does everyone think about this alternative?

http://www.ssbrakes....x.cfm?nPID=8999

 

For one, it's a cheaper item. I'd be getting a bigger booster (7 inch vs a 6 inch) and a new M/C. I believe it's newer technology used. Don't know what or how much modifications it would take to fit.

 

Just wanted to see what other thought about this? pros and cons.

 

thx,

Kyle

[yello620]

You can get a brand new one right now from rockauto.com for $45. Seems crazy to mess with a custom conversion when you can still get a new master.

[zenndog]

I am going to wade into these waters, the diagram looks like the ECI diagram and may be specific to vehicles where the master cylinder is mounted under the floor boards. This can cause the master cylinder to be lower than the wheel cylinders and the brake fluid may drain back into the master. That may be part of the reason for the residual valves.

 

I have a 1960 Bel Air that I was going to put discs on, the single master went out before I did the swap and I had to replace it. I wanted to put on a dual master but wasn't ready for the full swap. My friend who works on hotrods and custom cars gave me a part number for a 70's Chevy pickup master for a front disc/rear drum dual master. It was a bolt on besides running the new lines and works great to this day, with front and rear drums. The master cylinder is on the firewall, like a Datsun a couple of feet above the wheel cylinders. No check valves. No problems. But no discs yet either.

 

My 1957 International, with four wheel disc brakes, set up similar to the diagram above, using a corvette master set up for front and rear discs, and using residual check valves, has had nothing but problems. Soft brakes that pump up. I have bled them so many times that I feel like a physician in medieval times. The next step is to start plumbing around components to see if I can get the brakes to be solid.

 

My point being that maybe I have no clue as to what I am doing, and/or swapping brakes is a tricky thing.

 

Lastly, on my L320 with stock brakes, I switched to a 620 dual master for drum/drum combo and they have worked great.

 

Master cylinders and brakes are engineered for the cars they are put on and trading components can create new puzzles to be solved. Sometimes it just takes time and work. You will get a thousand different opinions as well, as always.

 

 

[zenndog]

metalmonkey has a similar thread....similar problem, but with the opposite circuit.......he exchanged the m/c on warranty and that fixed the problem. You might just consider doing that :)

 

Nothing like those Chinese auto parts, or any parts that have set on a shelf for a while.

 

 

 

[yello620]

My 1957 International, with four wheel disc brakes, set up similar to the diagram above, using a corvette master set up for front and rear discs, and using residual check valves, has had nothing but problems. Soft brakes that pump up. I have bled them so many times that I feel like a physician in medieval times. The next step is to start plumbing around components to see if I can get the brakes to be solid.

 

 

Master cylinders and brakes are engineered for the cars they are put on and trading components can create new puzzles to be solved. Sometimes it just takes time and work. You will get a thousand different opinions as well, as always.

 

 

Pretty sure we need some pics of the International. Any chance it has a Knox bed on it?

[hawaii_620]

I just picked up a used MC from a early 90s hard body at a junk yard. Looks to be a 1 inch diameter. I'm going to do whatever I have to do to make it fit and see how it works.

 

I know Hooiser77 tried a late model Frontier MC and said that it wasn't difficult to make it fit the 620 booster. But it was very hard to push the pedal (not his exact words). He didn't clarify what other components he was running... but did say he wanted to see how it would work with the D21 conversion. So I'm assuming he didn't have the D21 calipers at the time of his trial with the MC, and he never had a private message setup and was banned.... so we can't ask him any questions.

 

I have the Beebani setup and I'm going to try and see what happens. Hopefully I can get to it soon and report back.

[burrito213]

here is a magazine article that i have been reading over and over for the newbie to fully redoing this idea:

 

it comes from "classic trucks" and it is just informative how to for what they did. even though it is for CPP products, it still has worth if you are a little concerned about doing this project yourself.

 

 

 

i believe keep it stock is the way to go but some upgrades are important. braking safely, well, thats important.;)

[burrito213]

where can you pick up a 1978 Datsun 620 booster spacer?

 

is there a part #

 

if i can use this i will. is it possible to fit in a 320?