I have an 07 Expedition EL with about 43K miles on it and the other night my wife said the brake light came on and the next morning the brakes were essentially shot. I put about 1/2 quart of brake fluid in and within a couple of hours, the brakes were gone again. Because I'm out of warranty, I took it a local repair shop and they said the master cylinder is leaking fluid into the power booster and quoted me $900 to repair it!
I'm on the phone with my local Ford dealer and they are also quoting about $900 to replace both the master cylinder ($200 part) and power booster ($350).
What are your thoughts? Do you think the power booster does actually need to be replaced? My neighbor is a mechanic for a Ford engine plant and he thinks if you just clean it out good, that it shouldnt need to be replaced.
The booster could be cleaned out good IMO but you do need a new master cylinder. I priced both parts at newfordparts.com and the booster is $285 and the master cylinder is $143.
The brake fluid could ruin the seals in the booster if the fluid has been there a while. I guess better safe than sorry. If the seals get brittle or dried up and the booster looses vacuum, the brake pedal will be very hard to press.
I have never replaced either of these. I do know bleeding the system is crucial especially with the ABS/Traction control, etc. Maybe post this in the Brakes forum also.
FWIW, I am not sure if they are available, but you might be able to get a master cylinder rebuilt kit. Seals, etc. 99% of the time it's just the o-rings/seals that go bad, not the structural parts of the unit. I would bet the rear seal blew out and that's why fluid is leaking. You maybe able to get some local mechanic you trust to rebuild it and save a lot of money. Just a thought.