74' 620 locking up rear brakes.

[dimedriver]

Current setup.

280ZX 2+2 master 1 inch, I think but could be 15/16. I will double check

1975 Chevy K1500 front calipers Huge 9.94 inch piston. On Dana 25 axle.

Jegs brand proportioning valve between master and rear cylinders.

Stock rear cylinders I think.

 

When bleeding I can see the front calipers flex a little under the pressure, so they are getting good pressure.

The front brakes are built with new disks and rebuilt calipers. The master was a rebuilt unit from rockauto.

 

Problem: With the proportioning valve turned all the way in either direction the rears lock up before the fronts do anything.

 

I have not taken the drums off of the back ever. I may get to do this tonight. I will also bleed the system for a 3rd time.

 

 

So, My questions are as follows.

 

Could really rusted up drums + shoes cause bad wheel lock under good brake pressure?

Could the PO have put the smaller 11/16inch wheel cylinders from a front disk truck on a fully drum based truck lowering the pressure needed for lockup?

Am I crazy to even try to get the nissan master to push these huge chevy calipers(the chevy MC was 1 and 1/8 inch)?

Do I have a chance of getting a good feeling pedal(better that chevy stock pickup feel)? When bleeding it feels good but when the truck is running it feels mushy with the vacuum boost.

 

 

-Dime

[datzenmike]

Yours are 15/16" master.

 

Using a master that is smaller than needed will result in a longer peddle travel but effort is reduced. Like using a long lever. Using a master that is larger than needed will result in shorter peddle travel but more effort needed.

 

You could try backing the rear brakes off so that they have to travel further before touching.

[datsunaholic]

I think the problem is your master isn't big enough to move those huge front caliper pistons properly, even with the proportioning valve. And it's too BIG for the rear wheel cyls. The 280ZX master was moving front/rear discs, so it'll engage a drum cylinder instantly, irregardless of the aftermarket proportioning valve. Too many dissimilar parts.

[]2eDeYe]

Those are some big calipers to fill.

 

Dana 25? From a flat fender? That's ooooollllldddd!!! :lol:

 

Need a bigger master and bigger back brakes to keep up pressure.

[dimedriver]

Yeah the axle is really old. It had left hand lug nuts on the driver's side before I put the big brakes on.

The dana 25's came in kits to convert the early mini trucks to 4x4. I think they quickly changed to 27's and 30's.

 

Ok is there a bigger nissan over under mount Master that I could get my hands on? I thought the 280zx 2+2 was going to the biggest I think the 810 had the same one.

 

Could I put a cap on the rear outlet of the master so I can at least test the front brakes or will that make the master lock up because it can't put any pressure to the rear?

 

If the rear cylinders are stock they should be 3/4 which I can see as being to small for the a close to 1 inch master.

 

The rear axle is Datsun, I believe to be stock. But it some how has 5 lug's on 4.5 (old jeep pattern). I guess they did that to match the front axle. Long term Im going to need to get bigger brakes in the rear to keep them from locking up. Whats the biggest option there with out replacing the axle?

 

I do eventually want to do some crazy upgrades(diesel + dana 44's) on this truck but I would like to drive it a little 1st.

Should I just start searching for a Isuzu trooper rear 44 with disks in the right ratio?

 

 

-Dime

[albyneau]

I think the problem is your master isn't big enough to move those huge front caliper pistons properly, even with the proportioning valve. And it's too BIG for the rear wheel cyls. The 280ZX master was moving front/rear discs, so it'll engage a drum cylinder instantly, irregardless of the aftermarket proportioning valve. Too many dissimilar parts.

 

I cast my vote here as well. Those front discs will be great~ once you've got the rest of the hydraulics hammered out. One could spend a fair amount of time with a calculator to get precise flow requirements. But shootin from the hip, try looking towards other GM parts to "unbandaid the bandaid". A m/c from late 60's/early 70's disc/drum car will adequately serve those needs. Most wheel cyl's are basicly the same, so pester your local parts guy about his brake parts picture book~ and find the largest diameter wheel cyl you can fit. *THEN* start playin w/ that prop valve. You might even find some later model stuff that's metric, which'd be great~ cuz mix/matchin metric/SAE stuff can be one expensive can-o-worms!

 

Have you considered adding rear discs as well? That and a new m/c would be the BOMB...

[dimedriver]

I was hoping to stay out of the change every thing game but its starting to sink in.

 

Im pretty friendly with one of the parts stores in town that still has picture books;-). So I can start searching for slave cylinders there.

 

The Isuzu rodeo 93-04 and honda passport 98-01 (same "truck") came with a "metric" dana 44 under the back end. This axle has disk brakes with metric fittings on it. It may be an option for a brake upgrade and dana 44. If i remember correctly my truck has a 4.56 front axle. This ratio is available for the metric 44 but never shipped in a rodeo or passport. This is one possible solution to the bigger rear brakes cylinder problem.

 

 

Now for the master. I have never liked the way most chevy pickup brakes feel. I don't know if all my friends with chevy's just don't bleed their brakes or do chevy's just feel that way. I did look at pictures of the chevy master on rock auto and I can see it uses a left right mounting style. sorta looks like the modern nissan style. Has any one installed a GM master on a Datsun and if so how did they go about it? Could I do something as simple as an aluminum adaptor plate and a custom length shaft off of the brake booster? Are there any other Masters that may yield better pedal feel? Or should I just say this is a truck and the brakes are going to feel like truck brakes?

 

 

Thanks for the help everybody. If I make it out to the grage tonight I will take off a leaste one brake drum and take a look.

 

 

-Dime

[dimedriver]

1st gen Z cars have 7/8th wheel cylinders. which is ~16% larger in dia than the 3/4. But they don't look like they will fit. I looked at the 91' pick which looks like it may fit but has half the bolts at 13/16 inch.

 

 

Here is a pic of the my master and booster.

That braket or adaptor plate between the booster and master cylinder. Has anyone tried to remove that and put one of the more modern(side bolt mount) Nissan master cylinders there?

 

 

-Dime

[dimedriver]

ok I have a real good idea.

 

The 1999 nissan pickup Frontier I think its called, has a 1 inch master with the side by side mounting (only draw back) it also had 13/16 wheel cylinders that look like they will fit but use only 2 bolts.

 

 

 

Can some one with a modern nissan mesure the distance between the bolts holder their master on for me?

 

-Dime

[yello620]

I would be willing to bet that the master you have will work fine with the front and rear brakes that you currently have.

 

I think that you have issues in the rear brakes themselves, you mentioned that you had never had the drums off the truck. When these rear brakes are out of adjustment(loose), it will cause the shoes to bind and lock the wheel up when you apply pedal pressure. Get the rear brakes adjusted properly and see what happens.

The other thing i have seen happen with the rear brakes is that one of the pistons will be stuck in the bore, and because each wheel cylinder has two pistons, it forces the other piston to move twice as far, also causing wheel lockup. You can find this problem by removing the drum and watching the cylinder action while a helper LIGHTLY presses the pedal, you will have to hold one shoe at a time in place to see the opposing shoe move. make sure that the other drum remains on the vehicle while doing this, you dont want it to spit a piston out of the cylinder.

 

Just consider making sure that the stock rear brakes are working as good as they can before you start throwing even more mis-matched parts at it.

[dimedriver]

Thanks, Yello. I did go out to the garage but I was too busy looking up possible parts that may fit. I may get some time this weekend to pull the drums off.

 

I typoed the piston dia on the 1st post. They are 2.94inch which is huge.

 

So if cleaning up and maybe getting the drums turned and replacing the shoes does not work.

 

The 99' nissan pick up looks like it may have some parts that will work.

master is 1 inch (short of the chevy's 1 and 1/8 but the it may have a longer throw)

The front calipers were dual pot 1.64inch which is still dwarfed by the signal pot chevy

It has 1 inch and 15/16 wheel cylinders available in two different styles one looks very promising. The chevy wheel cylinder is 1 inch.

 

At the very leaste the wheel cylinders may work out. surprisingly the chevy cylinders also look similar but im sure they are quite a bit bigger.

 

-Dime

[yello620]

I would bet money that the chev wheel cylinder will not work on the datsun, completely different design.

 

Also consider this, looking for master cylinders. I am currently using a 280zx 15/16" bore master with stock rear drums and front calipers from a hardbody. Now these are a dual piston caliper, which also uses a lot of fluid. piston size is 1.685" x 2. Still does not quite equal the chev caliper, but it should be fairly close.

 

With all these issues, maybe consider putting a smaller bore GM caliper on the front.

[dimedriver]

I'm not sure if there is a different GM caliper that will fit. This is the sock caliper from a K10 which is the 1/2 ton 4x4 truck or blazer. I checked on rock auto and this is used up to the K20 and on some K30's 1 ton trucks.

The RWD trucks used a different mounting scheme.

Here is an outline of the upgrade I did to the jeep axle.

http://www.earlycj5.com/technical/brakes/early-cj-disk-brake-swap/

The swap uses IH disks and gm mounting plates with the gm calipers. There may be a jeep or IH caliper that will fit but it may require a special mounting plate.

 

 

I should get some time to mess with the truck on saturday.

 

-Dime

[dimedriver]

I got the passenger's side drum to come off. The adjust can move back and forth in that slot on the backing plate. Its really easy to move it back and forth too. I have not had much luck in taking the other side off. The back of the adjuster on the driver's side is different it has a screw in it. The passenger's side just has an arrow pointing to the front of the car.

 

I think I can get new adjusters from a parts store.

 

 

-Dime

[yello620]

I think i have a spare adjuster if you cant get them new anymore. The screw is there because somebody else has already Mickey Moused it together.

[dimedriver]

I have read in the FSM that it should have ~12KG of force before it moves Mine moves very easy so its not 12kg.

I have not gotten the other side off yet. I think the flat springs that hold it may still be available.

 

None of the parts shops had it. I'am going to check with a local nissan dealer that works with the racing crowd next.

 

 

-Avery

[yello620]

Here is a site that lists a little bit about caliper piston area's.

 

http://www.magnumforceracing.com/store4/wilwood_technical_information.htm#Calculating Caliper Piston Area

 

And wilwood makes a master cylinder that looks identical to the factory Nissan master and it has a 1" bore. Have not measured to see if the mounting bolts are the same though.

Second from the bottom of the page.

http://www.wilwood.com/MasterCylinders/MasterCylinderList.aspx

[dimedriver]

Are you talking about this one?

 

It does look very similar. I will have to measure one of the master cylinders I have laying around. I will not have a chance to do that until next week. :(

 

I have the adjuster fully adjusted in on the driver's side and have not had any luck at pulling the drum off. It also still has more drag when turning than I think it should. I have coated it with tons of PB Blaster so maybe that will make the difference after it has a few days to sit like that.

 

Know any tricks?

 

-Dime

[yello620]

Yes that is the Wilwood i am looking at. Awful similar.

 

The trick to getting the drum of is the size of the hammer. Accept that the drum needs to be replaced and quit hitting it like a girl.

 

If you are not using at least a 3 lb. sledge, you are wasting your time.

[dimedriver]

I'm using a 64oz framing hammer on the lip of the drum with an aluminum block. I will try and get someone else to hold the block so I can get a better swing.

 

 

-dime

[dimedriver]

Ok, got a sledge out and off came that damn drum.

 

I removed everything from the backing plate.

 

The adjusters on both sides are fucked up.

 

One side the back of that adjuster that goes though the backing plate was broken off and the PO drilled a hole threw it and put a screw in to fix it. This one was pushed all the way forward(removal position) and jammed in place. The other one moves if you look at it too hard.

 

 

I think the non-broken one is savable. I just need to clean the backing plate on both sides and put new springs in it.

 

That Rods "discount" datsun parts guy is close by so I will see how much he is charging for these parts. I will post some OIC's tomorrow.

 

I will have some new drums by tomorrow too.

 

-dime

[dimedriver]

Ok, Found a 75' 620 at pick and pull San Jose south. It had the adjusters but the adjuster shim springs were missing.

 

I took all the adjusters. Off of it.

 

 

Got back to the garage.

 

New shoes

New drums (normal 6 bolt lug pattern needs to be changed to 5 on 5.5inch)

New shoe return springs.

New wheel cylinders.

 

I drilled the drums out for 5 on 5.5 lugs so they would fit the 4x4 setup. Cleaned up the backing plate where the adjusters go. They now move more smoothly and with a lot more pressure. :) Yeah I don't need new adjuster shims. So I got the Passanger's side together and the the new drum does not fit over the new shoes. adjuster is pulled in all the way. :( So I start tapping the shoes up and down while pushing the drum on. It goes on but it was a real fight. Can not turn drum by hand or by standing on a breaker bar wedged in the lugs. So I go work on the other side. This time i don't tighten the wheel cylinder down until I have the drum on. It goes on pretty easy. and the drum turns will very little drag. Then I try driving the truck and it drags the passenger side wheel across the floor of the garage.

 

 

I jack up that side and take it apart. I change out the adjuster and put it back together and have the same issue but the drum is a little easier to put on. It still doesn't turn.

 

I think Im going to try these things when I get back to the garage tonight.

 

1- Swap drums from passenger's to driver's side. Pretty sure this won't help I tried it before banging it on there.

2- Swap one shoe from passenger's to driver's side. (I think this is most likely to work)

3- Swap to yet another adjuster.

4- have drum turned so its a little wider. (This should work but may end up with thin drum)

5- (bad idea)I hope I don't get to this. grind down shoe contact points a little. One near adjuster, other near wheel cylinder.

 

Do you guys have any other advice?

 

-Dime

[yello620]

De-adjust the parking brake a bit first. If a previous owner ever adjusted the parking brake to make it work better before adjusting the brakes, you will have this exact problem.

 

Also consider having the drums "checked" for size before having them turned. I have seen brand new trums that were several thousands under size. And that would cause the problem too.

 

Another thing to consider is that very few brake shoes actually are getting relined anymore. They throw the old away and stamp new shoes to put new lining on, of course these "new" shoes are manufacured to the tightest tolerances.

 

So, consider trying a completely different set of shoes first.

[dimedriver]

Thanks, Yello!

 

Im on my way out there right now. Thats why I wanted to swap one shoe from the other side in to see if I got two big shoes and two little shoes.

 

 

-Dime

[yello620]

Also realize that each shoe must go one in a specific orientation to work correctly. If not correct you will get a lock up problem.