Hi, I have a 74 f100 with manual brakes, rear drum, front disk, and I replaced the rear shoes and wheel cylinders, and the master cylinder, and a couple rear brake lines. Now after bleeding the brakes, after I drive for a bit, maybe 10-20 minutes the brake pressure gradually builds without releasing, and I don't understand why. Can air in the system cause this to happen? I did have a bit of trouble getting the brakes bled sufficiently, so I thought maybe that's all it is, but now I don't know any more.

Thanks,
Byron

 

A bad rubber brake hose can cause this.
Sometimes when they get old they brake up on the inside trapping fluid in the line causing the brakes to hold pressure after you let off the pedal.
Pump your brakes till they lockup then crack the bleeders one at a time.
The one that bleeds off the pressure will tell which hose to change.
Hope this helps.

 

You indicated it didnt start until after the new parts, possible wrong or bad master cyl. Take the cap off and watch for a little "geyser" to shoot out of the holes in the bottom of the reservoir while someone pushes the pedal. If not the pisons are sticking or the pushrod is too long

 

Sounds like the master cylinder pushrod is too long. If it's not releasing all the way, it's not uncovering the ports in the master cylinder and won't allow the brakes to fully release.

 

Ok, Thanks for the help, I'll check out those things today.
Byron

 

It seems the push rod was a little longer on this master cylinder than the one I pulled out, I backed off the brake light switch so the pedal could travel farther out, and so far it looks like it's working. It's funny how the simple things are the most overlooked!
Thanks again for all the help.
Byron