Power brake conversion issue

-
cudafish65

cudafish65

Well-Known Member
Joined
Jul 27, 2016
Messages
79
Reaction score
23
Location
long island NY
Converted my 65 barracuda to a dual master power brake system. 4 wheel drum. After about a 10 minute drive, all 4 brakes will lock up. I can relieve the pressure at the master and it will be fine for a little bit. I think I have a faulty master cylinder, has anyone experienced anything like this?
 
I believe that on a power brake setup that the push rod into the master cylinder is adjustable. If not adjusted right, I believe that it will cause problems like you have described. Searching for push rod adjustment should bring up a few be threads on adjusting it (sorry, can't find the ones I had bookmarked). Had a similar problem on my manual brake set up, but for different reasons.
 
I wouldn't assume it is the master cyl., it works fine cold for how long? As SIX225 stated, pull the MC loose from the booster and look at the pushrod, if it's adjustable that is one
option. Does the pedal get really firm when this occurs? Also check the strut link between the pedal & booster, is there a small amount of play, did it go on easy or....?
Also I assume You left enough air-space in the reservoir for the fluid to expand
w/the underhood heat etc. Unless all 4 sets of shoes are adjusted too tight,(not
common but possible), fluid expansion is the only other temp/time related item.
 
Last edited:
You didn't provide much info, what parts used, etc...
Fluid lines on engine side of master or fender side of master?
Exhaust heating brake fluid has been a issue for others.
 
Well the push rod is not adjustable, the lines run along the fender down to the stock location with a new distributor block. the pedal does get real firm when this happens. if there was too much fluid lets say, wouldn't it leak out the master? if there was that much fluid I feel I would have some sort of leak
 
The small push rod at front of booster going to master cylinder is not adjustable ?
 
Did you replace the MC? I ask because the power brake ones should have a retaining ring where as the manual ones have a plate with a dust boot on it and an undercut to retain the push rod. My car is a manual brake car where the plate was no longer flat and someone left the remnants of of the piston seals on the shaft . This prevented the piston from returning properly and resulted in locked brakes after driving for a few miles because the compensation ports were blocked.
 
SIX225 said:
Did you replace the MC? I ask because the power brake ones should have a retaining ring where as the manual ones have a plate with a dust boot on it and an undercut to retain the push rod. My car is a manual brake car where the plate was no longer flat and someone left the remnants of of the piston seals on the shaft . This prevented the piston from returning properly and resulted in locked brakes after driving for a few miles because the compensation ports were blocked.
Click to expand...

yea I replaced both the booster and MC as an aftermarket kit
 
Since cracking the lines open relieved the pressure, my research when I had my problem lead me to the blocked compensation port issue. Someone above mentioned loosening the master from the booster. Did you try that yet? That would allow the piston to travel back if the rod holding the piston in. Since the rod is not adjustable, maybe test it by shimming out the master from the booster with some washers.
 
That is very definitely a non functioning compensating port. To prove it release the fluid pressure by cracking a line. Very carefully measure the fluid level in the reservoir. Clean the rubber lid seal, the side that faces the fluid.Put the lid back on.
Now, go pump the pedal until it gets hard and high and no longer accepts pumping action.
Next go remove the lid. Look at the the seal you wiped dry. There should be fluid all over it from the returning fluid spray. Is it dripping? Next measure the fluid level. It should be exactly as you marked it. Is it? Put the lid back on and secure it.
Two no's means the fluid is trapped out there.
Let's get it to come back.

Oh, I see SIX225 already gots you covered!
 
I recall a 88 T-bird that would begin each day just fine then keep enough residual brake pressure to cause hot lining odor. The lady had the lining replaced a few times in a short period, front calipers replaced once also. Her son ( one of the salesmen where I worked ) said he thought she drove with 2 feet, riding the brakes. When it came through the door I couldn't turn a front wheel by hand. The next morning I could. I did suspect the booster but found the master cylinder was the fault. The spool valve/piston/whatever you wish to call it would stop just short of fully returned, as if the spring inside was weak.
The front wheel drive Cutlass Ciera that would lock up only 1 rear wheel is still the most notable oddball case in my memory. That was a pre antilock master cylinder proportion combo assembly.
I don't know that the T-bird came off the assembly line with a flaw. It had been in a fender bender.
I feel certain the lil Cutlass had its brake system flaw from day 1. Not all brand new parts are good parts. Good luck with yours.
 
Did you remove the brake lamp switch first? If not, the pedal may be bottoming out on it so the MC piston doesn't fully retract. If so, the fill ports may not get exposed. If correct, the tubes should be open to the reservoir with the pedal off, thus no pressure build-up possible. If not, you will not only lock-up pressure in the tubes, they will not refill so if even a tiny leak the pedal will keep dropping until you have no more brakes. Amazing how many people here don't understand the brake lamp switch and how to adjust it. It should not affect the pedal, only sense it.

Most Mopar factory power boosters have an adjustable output tip, as I recall. I would expect an after-market would be even more likely to have one. Unbolt the MC from the booster and look again. Same deal if it is too long, you will stop the MC piston from fully retracting. Adjust until you have just a slight amount of play, just like adjusting the brake cable on a road bike.
 
I had a Toyota truck do the same thing. I changed the m/c but that didn't help I sold the truck because it had over 300 thousand miles but often wondered about that problem. The only thing I could ever figure out is maybe where the vacume hose was attached the supposed to be vacume was positive pressure and it would pressurize the brake system. My pedal was rock hard. I might be way off base but it is all I could think about.
 
been busy last few days. finally got back to looking at it. as I was pulling the M/C off I noticed the car starting to move. I temporarily put washers in between master and booster and drove the car a bit. brakes worked perfectly. now for some reason my push rod is non adjustable so I had to shave it down a bit. now all works perfectly. thanks for all the responses
 
You must log in or register to reply here.
-

Back
Top