Brakes locking

[rc4mike]

I am in the middle of building a 70 mach 1. Had the original power bendix booster rebuilt and am using a 95 master. Cobra disk brakes front and rear with an adjustable proportioning valve and the factory distribution block. Car drives fine for half mile or so and then brakes start to lock up. They seem to not be releasing and only getting tighter as i step on the pedal. Gets so tight that car barely will move under power. If i open frotn bleed screw at caliper, all pressure released and brakes feel fine....for another half mile then starts locking up again. Any ideas?

Mike

 

[golf4283]

maybe the pushrod is the wrong size for the MC you are using. This would cause it to prematurely apply brakes even when the pedal is not depressed. did the MC fit snug when you installed it or did you have to tighten it down a lot?

on my '66 the MC fit all the way flush with the booster and all I had to do was tighten it down.

 

[rc4mike]

maybe the pushrod is the wrong size for the MC you are using. This would cause it to prematurely apply brakes even when the pedal is not depressed. did the MC fit snug when you installed it or did you have to tighten it down a lot?

on my '66 the MC fit all the way flush with the booster and all I had to do was tighten it down.

I am sure its the right size. Brakes dont start tightening down until i press the brake pedal. Its a progressive thing. Almost like they dont release fully each time i push the brake. This is a new thing. They originally worked fine (just installed system a few months ago).

 

[degins]

maybe the pushrod is the wrong size for the MC you are using. This would cause it to prematurely apply brakes even when the pedal is not depressed. did the MC fit snug when you installed it or did you have to tighten it down a lot?

on my '66 the MC fit all the way flush with the booster and all I had to do was tighten it down.

I believe this is the correct advise.

The booster pushrod must not touch the master cylinder piston when the pedal is at rest. If it does, the brakes will be applied a small amount at all times. As the brakes and fluid temp goes up from the friction, the hydraulic pressure will increase and the brakes will clamp down harder. This as a progressive process. You should remove the MC and readjust the pushrod so that it has at least 0.01" clearence and less than 0.05".

 

[68GEETEE]

two things come to mind, one is what are the condition of the brake hoses? The insides narrow up as they get old and wont let the fluid back when you release the brake pedal. Happened on my old buick.

The other thing is , are the calipers installed correctly, meaning are both bleeders at the top? I ask this because when I replaced my'68 mustang calipers, the parts house gave me 2 left calipers instead of a left and a right, I didnt catch the mistake until after days of frustration with my brakes locking like what you're describing. This left the bleeder on one caliper in the wrong position. Once I figured it out and got the mistake corrected, brakes worked fine. Hope this helps.

 

[rc4mike]

Well,looks like i had the pushrod too long. It was pushing on the master and slowly increasing the pressure. Shortened the pushrod and now they work great. Thanks for all the suggestions and help.

Mike