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Discussion Starter · #1 ·
hi,

i've got a problem with the brakes of my 76 3/4ton truck.
It's got modified brakes with the following modifications, the brake booster and front calipers are stock.

- rear disks (80-90 ford truck front disks) with 76-78 el dorado calipers (with park brake)
the rear disks were mounted with the tsmmfg-kit.
http://tsmmfg.com/2680.htm

- mp-brakes combination valve
the hold-off valve for the front disks was removed as per instruction from the manufacturer
http://www.mpbrakes.com/products/product-detail.cfm?product_id=614



- wilwood aluminum master cylinder 1 1/16" bore size (which i believe has no residual valves)


It's got the following problems:
- The brake pedal has too much travel and it's soft. Pedal travel ends 2-3" above the cab floor.
- mc spills fluid out of the caps (it doesn't leak though)
- The pressure point moves:
When the truck stood a few hours with the park brake applied, the pressure point is ~1" higher and the pedal is harder (4" from the cab floor)
After driving the truck for ~20-30 minutes the pedal is "low" again (2-3" from cab floor).

It does stop good though. All lines are dry, mc, calipers and combo valve are dry, i can't pump up the pedal. The pedal does not sink to the floor while pressing it.
The pushrod of the booster has the required 1/8" clearance.

This all makes me think, i misdesigned the system. I suspect it needs two 2lbs residual valves, one front, one rear OR a bigger bore mc.

I need some help with this. :-\

Thank you!

Regards,
Alex
 

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My first thought is you still have air in the system somewhere, and disc brakes shouldn't need a residual valve.
 

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Discussion Starter · #4 ·
s ǝoɾ said:
Or it has water that is heating up and boiling?
I think it's neither air or water. The brake fluid is fresh and clean (pushed several bottles through the system to clean the lines). Btw. the front calipers have new pistons and seals.

I've done some research on the residual valves: wilwood (and others) recommend them if the mc
sits lower than the calipers or if the vehicle has drum brakes.
Neither of the two is the case on my truck so i dont need them.

Then i stumbled across this page:
http://home.4x4wire.com/erik/4runner/brakes/

The author talks about hydraulic leverage (he swapped in a d44 with the small calipers, i have the
big ones with 3.125" pistons) of the brake system.

I calculated the numbers (7,67 sq." for the caliper piston, 0.88 sq." for the mc piston) and
this resulted to a hydraulic leverage of 8.7:1.

The stock mc (piston surface of 0.99sq.") would reduce that leverage to 7.7:1

The author claims that 8.7:1 leverage is too high, creating the spongy pedal.
7,7:1 reaches into the "good"-range but is still a bit high.
Couldn't find a bigger mc than 1.125" though...

Can anybody confirm that calculations and theory?

Thanks!

Alex
 

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Too big a piston would give a hard pedal, not a soft one.  Too small a piston would give a softer/easy pedal but go to the floor.  It's an issue of hydraulic leverage and I see you describing a different problem.  In little cars (like my perpetual Datsun project) we use just the MC in smaller stock sizes, but start adding power brake boosters when we get to 1" or larger (stock is 7/8) as an example of the harder pedal when MC piston size increases.
 

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Discussion Starter · #6 ·
hi doc,

i believe the pedal should be harder than it is at the moment.
There's the 1 1/16" mc installed and the pedal is softish.

I think installing the 1 1/4" stock mc (had three stock mcs blow out on me, two of them new ones  ::))
tales the pedal feel back to stock.
Hydraulic leverage is then back to stock specs at least for the front brakes.

If the pedal gets too hard i could still go for a 1 1/8" mc... On the other hand, my truck has power brakes that came with the 1 1/4" mc so even that should fit nicely.
Back to the roots that is... i hope the new stock mc wont blow out again.


I'm still concerned about the rears:

piston area:                            hydraulic leverage:
7.67 d44 caliper                      d44/wilwood: 8.7:1
3.24 eldorado caliper                eld/wilwood: 3.7:1

0.88 wilwood mc                      d44/stock  : 7.7:1
0.99 stock mc                        eld/stock  : 3.3:1

The ration between rears and fronts stays at ~2.3 (2.35 with wilwood mc, 2.33 with stock mc).
I dont know how to relate that to the stock rear drum wheel cylinders  ???

Alex
 

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You can try a larger piston, but that is a costly option to try with new parts.  If you have a good working stock one on hand, you can see the effect at no cost.  The part that makes me think it is an air-contamination issue is that the pedal feel changes as it gets hot.  I would think a mis-design would remain a constant problem.

My approach would be this - re-bleed MC (can be done with it still bolted to the truck by just directing tubes into the reservoir) then rebleeding the entire system all over again.  If no improvement, then the stock MC.  I would make sure to add an adjustable valve to the rear brakes so they could be fine tuned for discs.
 

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Discussion Starter · #8 ·
hi doc,

i'll go with that procedure. If it doesn't cure it: the new stock mc is on its way for ~90$.
The used stock mcs (and some cheap new ones) i had blew up on me (thank god they did that in the driveway).
I already have an adjustable valve at my place, in case the truck needs it...


The pedal feel doesn't change during heating up, sorry my description is incomplete:
It does change during using the brakes. It's hard right after i released the park brake
and gets softer the more i use the brakes.
No matter if the truck is running/not running, hot/cold.


Alex
 

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ok,those gm calipers need to be cycled with the e-brake lever,very similar to the buick regals which gave me tons of grief til i figured it out.crawl under truck with vise grips,lock onto e-brake lever and cycle back and forth while observing piston,you will see it crawl out of its bore til it will go no further.

reason its better right after using park brake,is it is slightly closer to rotor than before.
cycling the e-brake is the adjustment for the rear brakes.also check to make sure pistons lock into the pads correctly.they usually have dimples or something raised on the backing plate to prevent piston from turning.if piston turns,it wont adjust properly.
 

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Discussion Starter · #10 ·
offroader_dodge said:
ok,those gm calipers need to be cycled with the e-brake lever,very similar to the buick regals which gave me tons of grief til i figured it out.crawl under truck with vise grips,lock onto e-brake lever and cycle back and forth while observing piston,you will see it crawl out of its bore til it will go no further.

reason its better right after using park brake,is it is slightly closer to rotor than before.
cycling the e-brake is the adjustment for the rear brakes.also check to make sure pistons lock into the pads correctly.they usually have dimples or something raised on the backing plate to prevent piston from turning.if piston turns,it wont adjust properly.
Yup, that's right. Therefore i use my park brake quite often to keep them adjusted. But it seems they don't lock into the pads right...

I will check that soon, have to repair the recently blown trans first :mad:
 

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Discussion Starter · #11 ·
I'm afraid i have to pick up this thread again.
The tranny is repaired but the brakes are still of concern.

I noticed the mc pushrod does NOT have the aforementioned 1/8" clearance.
I'm going to correct that tonight.

But i'm afraid this will not solve the problem.

What is a good mc for a disk/disk-setup with stock d44 front calipers and big gm rear calipers?


Thanks!

Alex
 

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I didnt see whether or not you changed the proportioning valve but if not that may be the problem...disc brakes require much higher fluid pressure than drums. You can get a prop. valve for a 4W disc truck or an aftermarket one..Wilwood makes an adjustable one.
 

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I used a 4 wheel disc prop valve (same one pictured above) and a 76 Corvette 4 wheel disc master cylinder, bolts right up to the Dodge two bolt boosters. The 80's and up won't work because the pistons are different where the rod pushes them. You can change the piston to the one from your Dodge MC as long they both have 1 1/8" diameter bores.. I have a great pedal and no problems stopping my 44's..



 

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Discussion Starter · #14 ·
@440 PWXPRESS:
That's exactly the information i was looking for.
The 76 Corvette master is about the same $$$ as the Dodge master.

I installed a 1 1/8" master on friday and the truck has a nice and hard pedal now.
But the rears lock up too early. Does your truck do that too?

MP says, the combo-valve is adjustable (the do not anymore say that, but they did...).
I dont know how that's done, do you have further information on that?
What rear calipers do you use?

I did modify it to 4 wheel discs as per instructions...


@Doomdriver:
I'm running an MPbrakes combo-valve which incorporates a prop-valve. I have an adjustable prop-valve around but i refuse to install it until its evident that there's no way to adjust the MPbrakes combo-valve and the Corvette MC does lock up the rears to early too.

 

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Just about any time you retrofit disc brakes on the rear of a truck originally equipped with drum brakes you are going to need to use some sort of adjustable proportioning valve to keep the rears from locking up early.
 

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Discussion Starter · #16 ·
@El:
That's right and i try to accomplish that  8)
I'd like to make it look "stockish" though.

An adjustable prop valve (like the wilwood type) doesn't look stock... and it's officially illegal over here because one can tamper with brake bias on the fly.
If i can accomplish a good brake bias with smaller rear calipers or by swapping the Corvette master under the hood or adjusting the MPbrakes combo-valve and keep the looks stock i'm really happy and TUV (technical control board) is too which makes me even happier  ;D


When ordering the tsmmfg-kit i chose the big rear GM calipers in favor of the small ones as i was afraid, the rear brakes wouldn't bit hard enough on a 5000lbs truck.
I found that the big GM calipers instead are too big.


For the smaller (2 1/32") calipers (79-85 Eldorado, Riviera, Toronado and 80-85 Seville) (the big ones are 2 13/32") i need smaller adapter plates... (the big GM rear calipers have 7" spaced mounting holes, the smaller ones are spaced 5 1/2")

This is a fab thingy and moves the "smaller calipers"-solution further away and close to the adjustable prop-valve solution.



Btw.
What piston size are the front d44 calipers?

I'm not sure if i took my measurements right, before i ditched the old pistons (out of the rebuilt calipers).


Thank you all!


Alex
 

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But the rears lock up too early. Does your truck do that too?
No mine don't lock up, works perfect.. might be because of the added weight of the 14 bolt full floater hubs and wheel/tire combo but not sure..



MP says, the combo-valve is adjustable (the do not anymore say that, but they did...).
I dont know how that's done, do you have further information on that?
What rear calipers do you use?
No sorry kingcrunch, I said I was running the same prop valve but I think I'm wrong on that. I bought mine from Inline tube and it looks almost excactly the same as one shown above but I think mine was listed as a GM. I don't have a clue how to adjust it, I can't even find my papers on it now that came with it.. I installed it and just got lucky I guess.. I'm using front GM 3/4 ton truck calipers and rotors on all four wheels..



 
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