Brake pedal sinks to the floor at light

[bisman69]

Im having a problem with my 2002 Ford Expedition. The brake pedal sinks slowly to the floor at a red light. My mechanic replaced it with an aftermarket master cylinder and it did the same thing so he ordered one from Ford and its no different. Could it be a brake booster or is there a hell of alot of air in my lines? Help.

 

[ELVATO]

Have you tried bleeding the lines? Granted, I would have figured he would have done that when he replaced the cylinder, but figured I'd ask...

 

[Bremsen]

Yeh, m/c should have done it. No leaks anywhere? Try bleeding it again working from back to front. Booster works off vacuum, so if it had a leak the pedal would be hard (unboosted), not soft. I guess you could have a high vacuum issue to cause the system to be over boosted at idle (highest vacuum at idle). Still shouldn't go to floor though.

 

[GAINMOB]

since you changed the master...check booster...if thats good...bleed the lines again

 

[KWT2000]

did the master cylinder get bench bled befor it was installed?

 

[Biged1968]

if you did not bench bleed the master cylinder, buy a kit, to bleed it on expy, then bleed lines at both in and out on abs system, then bleed at each wheel, doing the farthest wheel from master cylinder first,
its better to have someone pump the pedal to bleed them then using a vacum bleeder, its something to do with the passages in the abs block
one little air bubble in the abs on these things can cause you fits

 

[andybeals]

One thing I noticed the last time I bled was that when my pedal-pusher pushed the pedal down, fluid would seem to be flowing back into the reservoir - is this normal or is my MC scrod? I've got "pedal slowly falling to the floor at stops", too. I'm considering putting a (bench-bled!) new Master Cylinder into it. That's one of the few parts in the engine compartment that's easy to get at!

 

[ZombieSoldier]

Check the brake lines themselves. If you have a line that is ballooning under pressure it will cause this as well

 

[silverbackexpo]

Ok I have a question wat if the lines were bleed but the and wasn't bleed will the petals still go to the floor.

 

[silverbackexpo]

Sorry wat I'm tryin to ask is does the and system have to be bleed on a 97 expedition.