Does Anyone Know The Answer???

[bisman69]

I own a 2002 Expedition XLT 5.4L. My brake pedal drops at a red light or stop sign and my master cylinder has been replaced. They bled all the air out of the system. What could it be? Power booster failing?

 

[smo0othride]

Sounds like a leak in the hydralic system assuming you are using the correct type and clean brake fluid. Even with the master cylinder replaced there could still be a leak somewhere else in the system.

 

[tonydiv]

If you aren't losing any fluid, I'm going to say that there's a leak in the booster. There's a check valve that is the most likely culprit for a leak.