Brake Problem Help Needed (revisited)

I still cant get a good firm pedal in my 51 Chevy. I installed a disc brake kit on it a couple months ago. It has a 7 inch dual diaphragm booster, a new disc-drum master, the correct residual valves and an adjustable proportioning valve on the rear. I have bled and bled and bled thee system. I seem to have all the air out. But the pedal still goes to the floor and the car barely stops. Its not even close to drivable.

So that else could cause a soft pedal and poor stopping other than air in the lines?


Here is a link to the original thread about this: http://www.jalopyjournal.com/forum/showthread.php?t=324167

 

Maybe I need a larger bore master cylinder. The master I have is for a 70-80 Camaro with discs. Does anyone know the bore size of that?

 

pedal going to the floor still indicates a leak/air in the lines. Any indications of leaks? Drips?

Also is there enough throw in the pedal? Is the plunger going all the way in?

 

Perhaps you could eliminate either the front or the rear and then see if you can get a hard pedal?

Could be bad residual valves

Gary

 

Can't you adjust the pedal throw on the shaft? Is it not going in far enough to press on the diaphragm? I may be way off, but, I had a similar problem with a project I'm working on. I too bled and bled and couldn't get a pedal. It finally dawned on me and once I adjusted my pedal all was good.

 

Buddy of mine bought a 54 chevy truck from a guy, that had a disc brakes on it. It had the same problem. When we got to looking at it, we found that they had put the calipers on upside down, due to buying the wrong rubber hoses.(to short) The bleeders were at the bottom insted of the top. We got longer hoses and flipped the calipers over, bled the brakes, and everything was fine.

 

The residual valves are new. I will check that the pedal is adjusted for the most throw on the shaft. The calipers and wheel cylinders are definatly on correctly. There doesnt seem to be any leaks.

I still think that I may need a larger bore master.

 

If you aren't getting any pedal and everything is adjusted correctly, a bigger bore isn't your issue. Maybe if you weren't getting enough pedal.

 

35chevy.com has the right idea.I had lots of problems with getting my brakes up and running. Just when I thought there were right they went soft. It is probably a combination of things. Block off the rear and then try it. I also bled using the jar and hose method and even though I thought I had all the air out, when I used this method I was astounded with the amount of bubbles I could see still coming out.

 

take the line off the master cy hold your finger over the hole and have someone press the peddle to see it you have pressure at the master cy. the last time it took 3 master cy before i got a good one for my sedan

 

Thanks for the replies. I should clarify that I am getting SOME pedal, but maybe only 10% of what it should be. I may have a bad master, or the wrong one. I bled the whole system back into the master with a long lose, at every wheel I pumped about 20 times after there was no more air. I am pretty sure I got it all, but maybe not...

 

I'll bet you still have air in your lines. Sometimes air finds high spots in the lines.

 

Manual brakes use a 1" bore, power brakes use a 1 1/8" bore. Which master cylinder do you have? If 1", you could go to the 1 1/8" and that would have a lot more volume to bring the pedal up.

Bob

 

Did the master come from a Camaro with power brakes? I STILL don't see a bigger bore bringing you up from 10% to full.

 

I am not sure if my master came from a car with power brakes. The part number is 10-1521. Did camaros have power brakes in the early 70s?

 

Yes, they did. Read my previous post. The 1 1/8" master cylinder has 27% more bore area than the 1" master cylinder. That's a substantial gain in volume when you're looking for reduced pedal travel.

Bob

 

Might be a silly question, but did you bench bleed the master cylinder?


Jay

 

Did you bench bleed the new MC before installing it, per the instructions? I've skipped that step thinking it wasn't neccessary, but NOOO, still had air in that wouldn't bleed out. Try a pressure bleeder, if you can borrow or rent one, they have a plate which clamps on the MC, and pressurized tank which forces fluid thru the entire system, that might work out.

 

All of the talk of size of mc bore is pissing in the wind. Don't matter. If the system is free of air, the pedal will be hard no matter what mc you have, unless the mc is bypassing.

Three people have asked you if you bench bled the mc before installing it.
Have you......................................?

 

I will third the bench bleeding idea. You may have done this a thousand times already. If so, sorry for the repeat. But if you haven't, remove the mc and bench bleed it. Recheck all your fittings in the system, put the mc back on, then do another round of bleeding. If you can get access to a pressure bleeder, as has been mentioned, so much the better.

Other than that, I've seen wonky proportioning blocks and valves do weird things in the past but it's much more rare.

 

"I installed a disc brake kit on it a couple months ago."

^^ what did the kit consist of? what kit did you buy? what brakes were on it before?

you speak of the other owners brake lines... ??? seems like when I did my 49 conversion I replaced all the brake lines as well. are you using the same lines from a single master system?

 

Isolate the problem. get 3 vise grips and lightly pinch off the flex hoses at the calipers and the rear end. The pedal should be hard as a rock. If it's not then bench bleed the M/Cyl until there are no more bubbles and you have a hard pedal Then release each vise grip one at a time and test the pedal to determine where the problem is. If it turns out to be the rear brakes, try adjusting the brakes some more. A poor drum brake adjustment can feel exactly like air in the lines. A low pedal that pumps up.

 

I thought you said it was part of a kit with a 7" booster? Is this all mounted below the floor with the original pedal, or is it a firewall mounted system? If this is mismatched stuff, you may have a problem with not enough "throw" to move the piston. If you're pushing the piston thru the cyl completely, boosted or not, you should be able (after bench bleeding it!) to get the rest of the system bled.

Somebody correct me if I'm wrong, but the mc shouldn't care if it's boosted or not, you can use any mc unboosted if the pedal ratio is correct, no? That doesn't sound like his problem here.

 

I found on my 58 that after bleeding my under floor master for hours with my daughter in the drivers seat that the pedal although moving at least 5"-6" to the floor the arm moving into the master was moving at most 1" - 1.5". Made a little bolt on bracket to extend the throw and bang brakes big time.

 

You could have a problem with the master cylinder by-passing internally. The part number you have is of a rebuilt master cylinder. I have had a few issues at my auto parts store with rebuilt master cylinders, more so with aluminum master cylinders than cast iron. God knows how many times the cylinder has been rebuilt.

Dan Marvin, Owner
Exeter Auto Supply

 

Wow guys thanks for all the responces.

First off I bench bled the master twice. Then I bled the system several times.

The disc kit is a ECI. It used 70's chevy calipers and rotors. The rear is stock drums. The pedal/booster is from Godman Performance. Its an underfloor booster/master, and has been on the car for years with no problems. The kit was for drum brakes, so I got a new master for it when I switched to front discs.

I hate to clamp off the lines going to the front since they are brand new steel braided lines that I had made. I am afraid of damaging them.

 

You can buy line caps instead of pinching lines.

 

That's a valid concern. Don't do it. If you want to isolate any part of the system, do it correctly by disconnecting the line and capping or plugging it with the appropriate hardware.

Bob

 

Even if you can't find caps, just double flair a 6" tubing with the right fitting and smash the 6" tubing flat,,,,,, redneck cap.

 

Hey, no problem, I'm stuck in the house recouping from surgery, and threw my back out yesterday as well getting up off the sofa, so I have nothing to do but post here, goof off on Facebook, and Hotrodders.com! T

That answers a lot of questions we all had. Now, are the calipers mounted with the bleeders up? I'm all out of ideas...unless you've tried the pressure bleeder thing yet...