I have a 1956 Ford F350 Dually, I installed all new brake cylinders, wheels and master. Bled the system, and the brakes work fine. In the morning, they appear to have weakened, and the petal goes almost to the floor, and pumping the brakes does nothing. But after driving for 5-10 minutes, they get firmer and firmer, and if I drive longer, they get so hard that the brake lights stay on and the BACK wheels, not the front, start to lock up to the point the truck wont move. The brakes are pretty simple, pedal goes to master cylinder with a pressure sensor on the back,.... WHAT am I missing ???

 

Is there some sort of vacuum booster? Sounds like a bad brake power booster on a newer vehicle. Just a thought.

Wally
1951 F1 - "Lucille"

 

No booster - but I did re-route the brake lines, and in just inspecting now - they are between 2-3 inches from the exhaust/muffler - they cold be heating up, expanding the hydraulic fluid - that makes sense - I'll try to reroute away frmo the exhaust

 

Hi. Did you replace the flex hose that is between the end of the hard line and the block on the rear housing. Some times they will claps inside and the fluid will flow under pressure to the wheel cylinders but won't let it return so each time you step on the brakes the pressure will build until the brakes lock up. When the truck sits over night the fluid will gradually weep back to the master cylinder. If you haven't changed the two flex lines on the front and the one at the rear you should do so.
Hope this helps.

 

I had that happen with an old Bronco that I had years ago. It turns out that the master cylinder had an internal compensator port that had failed. It was just enough pressure to have the brake lights stay on after a few applications of the brakes.

Dan

 

Check the recess in the new master, It may be shallower thant the one you took off. The push rod may need to be adjusted. Do you have the proper pedal free play? I usually go 1/4 to 5/16 at the top of the pedal. If the push rod is too tight, it will cause hard pedal and dragging brakes.