Can someobdy please help out with this. I have a 2003 F-150 4x4 with cruise control and 4-wheel disc. When I am at a stop light for a while, when holding the brakes, the pedal slowly begins to fade towards the floorboard. I have replace two master cylinders, the original and a reman and then put on a new Motorcraft one...all providing the same problem. I have been told by dealerships that it is my imagination or that this is a "normal" problem. I am sorry, but having my truck roll at a stop light while the brakes are applied is not normal. Has anybody else experienced this and what is the solution. Thanks

 

i would also have guessed the master cylinder. since thats it, i have another idea. i dont know how it does it, but the ABS can back off the brake pressure when the truck starts to lockup, even tho your foot is mashed the pedal to the floor. i assume the ABS is regulating the pressure and bleading it back to the master cylinder-- you might have a small leak in the ABS regulating valve?????

 

Thank you Scott for the reply....that does make since due to the fact that I am not experiencing any loss of brake fluid. Do you know where this ABS valve may be? I am hoping it isn't located where I think it is, underneath the ABS ECU mounted on the frame-rail. Do you happen to know the cost of the part. Thanks for your help.

 

Maybe the brake booster? Would that cause this issue?

 

I thought about that also, but everybody keeps telling me that the pedal would become harder to push if this were going out. Also, the dealer said that wouldn't cause it either....but I speculate it may since there is vacuum involved here and if it isn't operating properly, a loss of vacuum seems like a reasonable answer. WE'll keep trying

 

there was a good thread a couple of weeks ago about adjusting the rod behind the master cyl. took about 15 minutes and cured the mushy break sympton, not sure if this is exactly the same thing you are experincing

 

Most of the time if your booster lets loose pushing on the pedal is like pushing on the floor, it doesn't want to move. While extremely unlikely that you have purchased more than one bad master cylinder it is possible that the master is allowing the fluid to transfer from the front reservoir to the rear or vice versa. Also did you bench bleed the master(s) before installation because to me it sounds like the system need to be bled. When the pedal is low and you PUMP the pedal does it come up or stay at the same distance from the floor? If you pump the pedal and it comes up you have air in the system.

 

Fordsport
I am having the same issue, I changed the front calipers,the master and also the booster and still have the same issue. Somebody told me to check the rod length behind the master and it may make sense because I remember when I put the master on it slid on easy like the rod was to short and it wasn`t making contact with the master. I will pull the master off tomorrow and try it. I was also looking at that box mounted to the frame under the air cleaner hoping it`s not that.

 

FordSport2003
I took my master off and adjusted the rod on the booster so that when I put the master back on that it is about an 1/8" off the booster until I tightened the nuts and the pedal is up to the top and doesn`t fade at all.