Ok, I have a 94 F250 XL with 5.8L EAOD with 261K on lowrider.

Issue: Mushy Brakes. And ABS light is on.

Bought this vehicle back in April 2009. This vehicle had a number issues. Got it to "quasi" standards (driveablilty and safety). Majority of these has been resolved, but this one has not been conquered.

What has been done.

Inspected Brake system, checked to see if there was any leaks. Left rear wheel was saturated with brake fluid. Pulled wheel off, wheel cylinder was leaking. Please note, at this time, I could not replace the pads. Replaced wheel cylinder, use brake cleaner on the assembly, and "bled" the brakes. Still mushy brakes, pedal goes almost to the floor. Got me a Haynes manual, found out I have RABS system (1 or 2, I don't know) and that I need to bleed RABS valve. After bleeding that, and the rear brakes, still mushy, doesn't go the floor.

Took off the front wheels, right caliper had bleeder screw missing. In its place was a "screw", jammed into the caliper. Could not retap. Traveled to the "local" Pick n Pull found a decent caliper, no tears or leaks, plus, with a bleeder screw intact. Again, bled brakes, (in this order, RR,LR, RABS valve, RF,LF per Haynes manual) still mushy brakes. Plus right rear wheel "locks up", if I apply brakes hard. Note, this is with the engine running. And when I press down on the pedal you can hear a "hissing" from the power booster. Hmmmmm....

What will be done.

All new pads, and resurfaced rotors and drums. And then re bleed again. Now, I'm I missing something here? Master cylinder is not leaking. Do I have "a pressure differential valve"? Power booster leaking? If the power booster is leaking how can I tell, and what would be some symptoms that I would look for? I know there are knowledgeable members on this forum that can give me more info on this issue. I'm trying to avoid going to the "mechanic". I feel that I'm "technically" capable of conquer this issue. Any expressions would be greatly appreciated.

Q in Sac

 

There could still be air in the system. I think when the power booster leaks you will just lose fluid but it wont come out anywhere. I dont know if the anti-lock is so advanced as to cause a pedal loss to try to account for something but then again my truck is 6 years earlier. Hopefully someone else can speak up, it usually happens when someone else replies first (thats what I noticed) but I thought I would put in my two sense. Good luck!

{EDIT} Sorry, I did not mean the power booster leaks. I meant the master cylinder leaks into the power booster. Sorry about the mistake, I know a little more than that, heh. EPNCSU2006 helped me catch the problem when he said its separate from the master cylinder.

 

The power booster is independent from the master cylinder, so a leaky vacuum power booster doesn't necessarily mean the master cylinder is leaking. One sign of a bad booster is a vacuum hiss when the brake is applied. It shouldn't affect how the brakes operate, though, as long as you are still getting the power assist - it's just a vacuum leak.

A leaking master cylinder can be disguised by the power booster if the vacuum pulls the leaked fluid to the engine. The master cylinder can also have an internal leak where the piston o-rings don't seal well with the bore. This will cause a sinking pedal, but it should be firm at least for the first pump. Is the pedal feel different if you make a quick stab at the pedal or just ease into it?

My guess on the locking right rear is that it is being caused by the imbalanced friction between the oil soaked shoes on the left and the dry shoes on the right.

 

You're going to have to redo the rear brakes, no ways about it. Once the pads/lining is contaminated with brake fluid, they are done for.

Did you deplete the entire old brake fluid? I bet it has never been changed. Old moisture-absorbed brake fluid can cause soggy brakes.

 

I have nothing to offer on your problem just a question.

What does "Q in Sac" mean?

 

EPNCSU2006: Master Cylinder? How can I tell? What are the symptoms?

Jermafenser: I will redo the rear brakes next week with new pads and resurface the drums. Old fluid has been replaced.

qman: 1st initial in my 1st name, "Sac" is where I live, Sacramento.

 

A master cylinder with an internal leak will have a slow, sinking pedal, but should be firm when the pedal is pressed quickly. If the pedal is pressed slowly, pressure will bleed past the o-rings faster than it builds in the lines and the pedal will just go to the floor.

 

Do I have "a pressure differential valve"?

Nope.

Still mushy brakes, pedal goes almost to the floor.

Is it truly "mushy" feeling, did or has the pedal ever "pumped up" during bleeding? or does it just have excessive pedal travel?

Did you make sure the rear shoes are adjusted until they lightly drag on the drums?

 

It is a "slow, sinking pedal". Not firm when quickly pressed. Time to replace master cylinder. Thanks, man. I appreciate it very much. Out.

 

If the pedal is not hard if you give it a quick pump, double check the rest of the system to rule out the possibility that the problem is somewhere else in the system. You're not losing fluid, are you?

Be sure to bench bleed the master cylinder before you install it into the truck, or you'll never get the system bled.

 

Pedal is not hard. Even if I pump several times, its mushy. When you press on the pedal, you hear "shhhhhhhhhhhhhhhhhhhh". I double checked the system, no leaks anywhere. But I will replace the shoes later this week and maybe the MC.

 

What is the condition of your rubber brake line hoses? These can get very old and expand enough when pressure is applied that the brakes never get pressure until the lines stop expanding.

If you have a cruise control option in your truck, there is a vacuum switch that dumps vacuum off the cruise control when you hit the brake. Be sure this isnt the vacuum hiss you are hearing.

 

Brake lines look OK, as I can tell. They probably need to be replaced also. I have an aftermarket cruise control and it doesn't work. I will look and plug that vacuum line.

 

I dont know about the aftermarket cruise control but the Factory Cruise did have the vacuum dump switch behind the dash that worked with the brake pedal.

My brake hoses looked fine but after messing with them I found the outer rubber cracked all the way to the core. When I removed the hoses, the insides were disintegrating from years of brake fluid.

 

I have an aftermarket cruise which has vacuum resiover not an issue.

Since it's a 94, 15 yrs. I believe its due for a replacement. It looks like MC and brake lines replacement.