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Brake issues

Boostwerks.com said:
Frank, check the gap between the rotors and pads. After the pedal is released the pad should still be making contact with the rotor, otherwise the caliper is retracting too far.

If the system is air free and the pads are touching, a larger MC would be next on my list.
Click to expand...


Frank Z's problem is very perplexing. From what I understand is that the pedal goes to the floor when it is held as in a fluid leak (see post #70). Short of being there and providing a second pair of eyes I am about out of ideas.

My theory is, if the MC is too small, a few strokes should/would build pressure and it will hold as long as the pedal is held down. The next application of breaks will require the same procedure ie; a couple of strokes then hold the pedal. If the pedal is loosing pressure and then one would thing that there would be a leak internal or external. All the parts that have rubber that would be a suspect have been changed several times IIRC and there is no evidence of fluid leaking on the outside. Does the fluid level in the MC reservior change?

Who knows, maybe a larger master cylinder will solve this mystery.
 
I originally had a stock 15/16 XJ MC in mine, with stock '88 front discs and Cadillac calipers in the rear. Once properly bled (the Caddy calipers can be a bitch!), my only problem was a pedal lower than I preferred. Too small of MC WIL NOT result in a pedal going to the floor, unless the calipers are HUGE compared to stock. Low pedal, yes, but firm, if properly bled.
I latter upgraded my MC to a 1" bore. Then I upgraded my front calipers to '79Chevy, which have a much larger volume requirement than the OEM XJ's. My 1" MC--which I believe is the same size as the ZJ MC, worked fine, except, again, due to need for more volume of the new front calipers, resulted in a pedal that was too low for me--but "firm". No way did the 1" MC result in a pedal that "slowly" leaked to the floor. The MC piston size will not cause that to happen.
Being able to "pump up" the pressure is a sign of air in the system. The only way the pedal will settle while under constant pressure, is a leak--external, or internal.
 
Drove the heep out to falcon (about 20 miles) yesterday and visited some friends that are also running Vortec motors and non-stock axles. We went over everything that I've done so far and what the results have been. They've run the same brake system (TJ's actually) and didn't upgrade their M/C's till they went from 44's to D60's front and rear.

So I'm gonna have to try a new M/C with a larger bore.

I'm going to head down to the Pull-n-Pray this week and scour the yard for a few options.
F250
E350
Durango
Kenworth
Mack
Peterbuilt
Freightliner
Yugo......

More "Crying Time" posts to follow.....
 
The 1.250 is too big for your system. One thing you will accomplish is to once and forever quash the notion that your problem is caused from a "too small" MC; your volume will now be excessive, dropping hydraulic pressure to your calipers significantly. Chevrolet used a 1.125 MC in the '79 1/2-ton truck, and a 1.250 in their '79 3/4-ton truck (note that "1/2-ton brakes were also available in the 3/4-ton truck as well, so just because the axle came out of a 3/4-ton truck doesn't necessarily mean the brakes are "3/4-ton"). The front calipers used with the 1.250 MC had 3.146 OD pistons, which is ~7% larger volume than the 1/2-ton calipers pistons.
I basically have the same brakes you do (front Chevy 1/2-ton calipers with 2.934 OD pistons, and rear Caddy "metric" calipers with 2.121 OD pistons), and am using an '03 Dodge Durango 1.125 MC bolted to an '01 XJ booster. The pedal is mid-to-high and firm. In fact, almost too firm; a 1.0625 (1-1/16) MC would probably work best with "1/2-ton" brakes. Remember, the truck your axle came out of had a much larger booster in it, allowing use of the bigger MC. Manual brake systems always had a smaller MC in them than did the same model with "brake assist".
 
techno1154 said:
From what I understand is that the pedal goes to the floor when it is held as in a fluid leak (see post #70). Short of being there and providing a second pair of eyes I am about out of ideas.
Click to expand...

this would indicate an internal leak in the master cylinder, the fluid is being pushed around the piston instead of being compressed by it. Rebuild or replace the master cylinder. it's not uncommon for one to be bad out of the box, especially if it's a reman unit.

take the lid off, and have someone pump the pedal rapidly only about an inch or so, if there is air in the master still you'll see lots of little bubbles (think soda pop).

edit: I just read back through and saw you replaced the master multiple times. nevermind that then, heh. still sounds like air in the system to me.
 
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congrats, Frank! I'm dumbfounded why you needed a bigger MC for that application based on my own experiences -- makes me wonder if Grimjeeper isn't onto something with the internal MC leak theory -- but I'm really glad it worked! :cheers:
 
xj-grin said:
makes me wonder if Grimjeeper isn't onto something with the internal MC leak theory -- but I'm really glad it worked! :cheers:
Click to expand...

you can test a master cylinder for internal leaks by picking up brass flared plugs and plugging both ports on the master after it is bench bled. if you can push the piston in with the ports plugged or it slowly goes in with constant pressure it is leaking internally.

I'm glad you got it worked out, I hate brake problems. we went through 5 master cylinders on our shop delivery truck a few years back before we finally got a good one, auto parts just aren't made like they used to be :bawl:
 
xj-grin said:
congrats, Frank! I'm dumbfounded why you needed a bigger MC for that application based on my own experiences -- makes me wonder if Grimjeeper isn't onto something with the internal MC leak theory -- but I'm really glad it worked! :cheers:
Click to expand...
I'm just glad it stops now.

Joe94xj said:
Good to hear you got it working.
Click to expand...
thanks again for all your help Joe.

GrimmJeeper said:
you can test a master cylinder for internal leaks by picking up brass flared plugs and plugging both ports on the master after it is bench bled. if you can push the piston in with the ports plugged or it slowly goes in with constant pressure it is leaking internally.

I'm glad you got it worked out, I hate brake problems. we went through 5 master cylinders on our shop delivery truck a few years back before we finally got a good one, auto parts just aren't made like they used to be :bawl:
Click to expand...
A real head and gonad scratch-er to be sure. But...All's well that stops well!
 
I would say it was just too small for some reason. If the master was bypassing pinching off the lines would have not give him a hard pedal like had when the lines were pinched.
 
HEY FRANK GLAD YOU THAT ALL FIGURED OUT!!
now it's my turn please. Finished everything on my full width HP 44 with chevy disk outers and full width big bearing ford 9 rear and I am having fun fun brake issues. I've been following franks thread and hoped his fix would be mine too. But I think (from what I read) that I would not need a larger master cylinder cause I'm still (hopefully temporarily :rolleyes: ) drum in the rear. I can bleed everything from bench bleeding the MC - vacuum bleeding the proportioning/load sensing factory valve- and all the wheels in the correct order (RR LR RF LF) I get a stiff pedal initially (without engine on) but then it fades out. And with the engine on it sinks right to the floor. Do you think I might have the same issue as frank?
I have bled and bled and bled and bled the brakes. I did it manually with frien and pedal- vacuum thru the bleeder valves and pressure bleeding thru master cylinder cap contraption with 15psi.
 
I'll just go ahead and get that one and give it a try (I bought a new one for my xj and read all the swap threads after installing it but since I basically live In and sign over my paycheck to autozone they said I could swap it out for the Durango MC.

THIS IS MY FIRST REAL EXPERIENCE WITH BRAKES AND I LOVE WORKING ON THEM :flamemad::flamemad::flamemad:
 
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