I recently changed my rear drums, for a week or so the braking was fine. Now if I press the brake peddle down slow, when I'm going at slow speeds the peddle goes almost to the floor but still stops. If I hit the peddle fast it will have pretty good pressure. I am not losing any fluid in the master cylinder. Could this be a bad master cylinder or brake booster?

94 F150
manual trans
4.9l

 

When the seals wear out in a brake master cylinder the fluid doensn't stay where it is supposed to but also doens't leak on to the ground. The piston in the master cylinder forces fluid to the wheel brakes when you push the brake pedal. If the piston seals in the master cylinder are bad then some of the fluid that should go to the wheel brakes leaks backwards around the piston seals. If the seals are bad it first shows up when you are pushing on the brake pedal for a long period of time. If you push the pedal hard you stop quickly and may not notice the brake pedal keeps going.
Start your truck and hold steady moderate pressure on the brake pedal to see if it slowly goes to the floor. If the pedal does slowly go to the floor then replace your master cylinder. Be sure to bench bleed before installing in the truck and don't bottom out the piston.

 

Thanks for the help. I changed the master cylinder a few hours ago and bled the brakes, its alot better now.

 

How tight did you set the rear brakes using the adjuster? I like to adjust it until it just drags when I turn the drum with both back wheels off the ground. If there is too much travel on the drum's cylinder you will have to push the pedal a lot farther down.
Do you set the parking brake? how often?
What level brake shoes did you buy? cheap, med, lifetime?
It seems like the cheap shoes wear really fast. You could just need to adjust them again.

 

The rear brakes are alittle tight still I need to back them off a bit. I picked up cheap pads, this is my beater truck it doesn't do much driving. As far as I can tell there isn't an adjustment for the parking brake.

Anyways replacing the master cylinder cleared up the problem. Now if I could only figure out the thumping in the front end.

 

Also guys don't forget about the RABS valve mounted down under the drivers floor on the frame. Basically all it is is a pressure accumulator. I chased a similar problem on a 92 Explorer. I changed everything just to find out it was the RABS valve. What happens is old, rusty brake fluid will cause the valve and seat of the valve to hang open and allow the normal pressure of the brakes to fill the accumulator instead of going to the rear brakes.

If you can "pump up" the brakes and they work it's probably the RABS.

Also, since you changed the MC did you bleed the RABS when you bled the other wheels? Make sure to bleed it too. The bleeder screws right on the top of it.

 

Nope I didn't even think about the the RABs, I'll do that this weekend. I'm getting awesome pressure now without having to pump the breaks multiple times, its just there.