Brake pedal to the floor

[JEyster]

2006, fast hard push on the brake pedal (like a panic-stop), I hear air hiss under the dash and the pedal goes to the floor. Under normal braking (even hard-brake, just not a fast push), all is normal. What am I dealing with?

 

[DieselMonk]

Are you losing brake fluid?

 

[dkincaid32]

2006, fast hard push on the brake pedal (like a panic-stop), I hear air hiss under the dash and the pedal goes to the floor. Under normal braking (even hard-brake, just not a fast push), all is normal. What am I dealing with?

Definitely the master cylinder. There was recall about them failing not long ago.

 

[SafariGoneWrong]

2006 XLT 4WD here. I went through this about a year and a half ago and still have imperfect braking. Symptoms were pedal slowly went to floor when engine running but firm pedal once there was no vacuum to the booster. I was convinced based on previous experience the rear brake ABS circuit (mine is a lowly XLT, only one circuit to the rears) dump valve in the ABS valve assy was leaking. Replacing ABS valve assy offered no change. Replaced master cylinder with no change. So I took this as an opportunity to replace all calipers, flexible lines and even the hard lines from the master cylinder to ABS valve assy. No change. Beware of bad new master cylinders even from known brand names. My first replacement master cylinder from RockAuto.com leaked out the rear seal while bench bleeding. I'll be replacing the master cylinder again soon with another brand...what else could it be? No leakage...no rust on brake lines anywhere...Texas and Georgia vehicle.

 

[Prcyfnl]

same thing happen on my 2005 xlt this year, I replaced brake booster and master cylinder and now brakes better than ever