I have a '00 F150 5.4L Lariat edition, was driving today when all of a suddent I found my brake pedal going to the floor and the truck taking a long time to stop. The tricky part is that when you first hit the brakes it's solid but as you're coming to a stop it suddenly stops resisting and sinks to the floor. Also if you pump it a couple of times it goes straight to the floor without resistance.

I limped it back home, and don't see any leaks though I did not take a very good look yet. And I also don't think that air could have gotten into the system because it was working fine in the morning when I left home.. nor have I done any work to the brakes lately. Any ideas as to what could cause this?? Thank you!

 

Sounds like a master cylinder going bad.

 

x2 on the master

 

Thanks guys... is this a common problem?

 

a master cylinder will fail at some point, common no, but not unexpected.

do you see any fluid on the brake booster? sometimes the rear seal of the MC will leak and it drips out between the MC and the booster.

Do the brakes "pump up"?

did you check fluid level?

any signs of a leak at the wheels?

rule out the simple stuff then go for a MC replacement.

 

I agree with Neggy - definitely check the entire system before you assume its the master cylinder. It might be a flex line leak or a leak at a fitting.

If you can't find any external leaks - then I'd change the master cylinder.

BTW - if the master cylinder is leaking out the rear seal it would leak INTO the booster, not on the outside of the booster. There is an O-ring to maintain the vacuum in the booster so the fluid can't "drip out". If you think the rear seal might have leaked, use a flashlight to look into the booster (after removing the MC) and see if there is a puddle of fluid in the bottom. (you can also dip a strip of rag or something and see if it gets wet)
If there is fluid in the booster, you should empty that out before putting the new MC on.