Sinking brake pedal

Asked by Horseaddict1960 Jan 21, 2019 at 07:21 PM about the 2011 Ford F-150 XLT SuperCrew 4WD

Question type: Maintenance & Repair

Why is the break pedal sinking to the floor. The  
calipers don't leak checked for leaks no brake fluid
leaks master cylinder still full of fluid. Just started
doing it today. Can’t get it into Ford service for a
few days.... local mechanic thinks maybe vacuume
pump but not sure.

1 Answer

74,210

I doubt it is the vacuum pump, but could be. The piston inside the master cylinder has two "O" rings which prevents brake fluid from passing backward behind the piston when pressure is applied. Damaged rings could cause this problem. The rings are often damaged with brake service is performed and the person doing the job has someone pump the brakes while they are bled. The piston inside the master cylinder never travels it's full stroke when in use, even emergency use. Pressure prevents this. Therefore, corrosion can form at the further end. If the rings come into contact with the corrosion, they can be damaged causing a backflow of fluid, causing the pedal to go to the floor.

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