54 Chevy disc brake questions

W

wisch79

This is a little off topic but thought some of you may have an idea. I am working on a 54 chevy car that has a Mustang 2 front suspension and the stock rear. This car was done at another shop and we are trying to fix some problems with it. It has disc brakes all around, non power dual port master cylinder, 2PSI check valves in the lines, a proportioning valve in the rear line, air ride, and all new rubber lines at the front and to the rear axle.
The problem is, you have to pump the brakes 5-7 times before you have good pedal. It will hold this pressure while you have the brake pedal pushed, but let it set 10-15 seconds without pressing the pedal, and then the pedal goes to the floor and you have to pump again to get pedal. We bled the brakes and it shoots fluid with no air. Any idea what would be causing this or something to check? We are stumped! Thanks!
 
wrong master cylinder and the adjustable proportioning valve belongs on a dirt track car. get a manual corvette master cylinder and a real proportioning valve.
 
Thanks Shine. I do not know what master cylinder it has now, but will look into that. What year/years should I look for? Another thing I just thought of is that he left the original master cylinder in place as that is what the pedal bolts to. Then a bracket is made off the rear of it and mounts the new master cylinder. Sounds wierd and looks wierd, but that is what he did. The original is gutted and a rod goes through it to actuate the new master. Another thought we had was the rod was the wrong length. Any thoughts on how to determine if it is the right length?
 
i would run backwards from a build like that. you can buy a floor mount bracket / master cylinder from eci or master power brakes. a proportioning valve from a dual disc car will work.
 
Thats the hard thing for us to grasp. He spent a lot of money on this car. M2 bolt in front kit, nice 4 link rear, Wilwood calipers, Wilwood check valves, all decent stuff, but didnt buy a bracket for pedal and master cylinder. Really weird! It just baffles us that you can get pedal by pumping, but loose it after it sits a short bit. No leaks we can find and fluid shoots out with no air when we bleed the brakes. Just doesnt make sense where the problem is.
 
the master cylinder does not move enough fluid on a single shot so you have to pump it. the one on it is likely for disc drum . if dual disc both reservoirs are equal.
 
I'm with Shine on the most likely wrong set up but there is one other thing to check that can also cause your symptoms.
When they converted and installed the rears, it is very common to not have the disc's shimmed to proper spacing.
This is done with an Axel plate spacer or round spacers but sometimes what comes with the kit is not enough and you will need to do final adjusting with a grade 8 washer.
Check and see what the play is with the rear pads also an emergence brake pull can be a giveaway depending on type of set up.

If the disc's with no peddle applied is sloppy, it is also a shimming problem.
 
i agree with shine, sounds like a bad master cylinder. my 40 coupe did the same thing once. also agree that the "mickey mouse" setup used needs to be scrapped & redone correctly. the correct mounting bracket/master cyl assembly isn't expensive, the trainwreck waiting to happen can be.

here is THE best brake reference material i have run across in one place. i reference this material fairly often just to double check what i plan for a build:

http://www.hotrodheaven.com/tech/brakes/brakes1.htm
 
myself i would loose the under car stuff and hang an s10 setup on the firewall. i use it all. pedal to pvalve. going together 1.jpg
 
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