Strange Issue with Brakes

[Detroit Iron]

I started having a strange issue with my brakes and wanted to get input before I start throwing money at parts.

I did the Richard Ehrenberg Disc-o-tech upgrade on my '64 Polara. Basically it's big later model B-body discs up front with the pin mounted calipers. Rear brakes are 10" drums, and I have a Mopar Performance aluminum master cylinder.

When I begin driving the car the brakes function properly. About ten minutes into the drive my pedal goes rock hard, and the brakes seem to be applied slightly. After driving this way for about 10-20 more minutes, the problem will go away, only to return a few minutes later.

At first I figured I was getting vapor lock in the lines, so I bled the system; fully purging all the old fluid. The problem still exists. At this point I feel the problem is with the master cylinder. Maybe the piston is hanging up. Maybe it's one single wheel cylinder. Any thought on this situation would be greatly appreciated.

 

[patrick66]

A great tech tip that my friend told me addresses this very thing. He is the auto tech instructor at the local vo-tech. My Coronet was doing the exact same thing. The front discs were maintaining a certain level of pressure, even though the brakes were NOT applied. When the brakes were applied, they became very touchy, and feeling quite hard on application.

The solution involves a drywall screw and a pair of pliers. Confirm which circuit in your master cylinder is feeding the front brakes. Remove the brake line at the master cylinder. Screw the drywall screw into the hole in the master cylinder brake line hole. Yank out the residual valve, as this is the problem. Reinstall the brake line and bleed the fronts. Test, and then go for a road test. Your problem should be resolved.

BTW, I replaced the calipers first, thinking they were bad. They were not.

 

[Cranky]

Yup, disc brakes do not use a residual valve....only drum brakes need it.

 

[Detroit Iron]

So just to wrap up this thread. During brake bleeding I found the pedal wasn't returning all the way. The piston was hanging up. So.... I simply replaced the master cylinder. I got a Strange Engineering. The problem is gone. Bye, bye...

Thanks for the input on this!