I have a 91 351 F-250, 2 wheel drive, with rear wheel ABS and 133000 on the clock. For years I have been putting up with a brake pedal that slowly sinks away, particulary if you vary the pressure while braking. Several years ago I replaced the master cylinder with no effect. Recently I had both the front and rear brakes completely rebuilt, with new cylinders in the rear.

Braking is good, but the sinking pedal leaves you with a sinking feeling, especially when you are making a "high performance stop" in heavy traffic. When I have a chance, I can pump the pedal, cycle it to the top and press again and get a higher pedal.

Sounds just like a bad master cylinder, but I swapped that already. I've been suspicious of the RABS valve, but I'd like some advice before dropping $100 on that with no hard evidence it's the culprit.

In another post, I read a discussion about a bad brake booster leading to all sorts of mayhem. Could this be it? When I step on the brake pedal, I hear a hiss from the pedal area, which stops when the pedal travel stops.

If anyone recognizes this problem, I sure would appreciate a reply.

 

That hiss sound is most likely thr power booster. I would go get one & quick. When it finally blows, you will be mashed to the floor with no stopping power what so ever (Been there, done that)

Walter

 

if it were the booster would'nt the hissing sound be there as long as the pedal is pushed down? he said it stopped when pedal travel stopped. also if the booster went bad you would still have brakes, they would be harder to use... kinda like a vehicle with no power brakes. you just would not expect that. the booster is totaly separate from the hydraulic system, it just allows for easier braking, so save your money i dont think its the booster. i could be wrong. i dont know about the rabs. it could be the master... i have gotten bad master cylinders before. just my 2 cents.


87 f-250
" if you are going to have a bear, it might as well be a grizzly"
4.9 granny tranny 4.11 gears

 

I did the entire system because of a very similar problem.
Master cylinder twice, complete front and rear brakes.
Booster checked out. My problem was in the RABS valve. My pedal would drift but not all the way to the floor. If it drifted when I was moving, I would pump it up and it would still drift back down a bit until I came to a stop. If I came to a stop and pumped it up again, the brake pedal would stay pumped up until I released it to move foward. My truck is a 92' F-250 w/ rear ABS.
It could be different from your F-150. RABS valves can be costly. I believe I paid $150.00 for the valve and another $160.00 for the core. Good luck. Jake.

Jakegypsum // Ford fleet, soup to nuts.

 

There is a Tsb dealing with the problem you describe .They use Phenolic caliper pistons ( Same material 300 engine cam gears are made of ) & heat causes them to create excessive pedal travel . The only resolution is to replace the calipers .

 

Thanks for this information. I was looking for some sort of confirmation it might be the RABS valve before spending money. I've also heard about the phenolic caliper pistons.

Is there any way to tell which of these possible fixes to try first?

 

the cheapest

87 f-250
" if you are going to have a bear, it might as well be a grizzly"
4.9 granny tranny 4.11 gears

 

My RABS problem was one year ago. My calipers were replaced approx 3 to 3-1/2 years ago. Maybe this can be helpful. Good luck.

Jakegypsum // Ford fleet, soup to nuts.

 

Thanks -- going back and reading your previous message, and thinking about it, I think you are describing my truck's problem just about exactly right. Looks like I'll swap out that RABS valve.

Thanks to everyone for their help. There is no substitute for experience.

 

I hope it all works out. Good luck. Jake.

Jakegypsum // Ford fleet, soup to nuts.