F-150 brakes solid when engine off, but go to the floor when on.

Asked by 84dna Jul 21, 2020 at 03:50 PM about the 2014 Ford F-150 FX4 SuperCrew 4WD

Question type: Maintenance & Repair

2014 Ford F-150. Part of master cylinder recall. Have replaced the master
cylinder 2X. Checked the Power booster, not an issue. Checked the ABS
modulator, it is fine. When I push the brake with engine off, very solid. I'm not
losing brake fluid. When I turn the engine on, the pedal sinks to the floor, and I
have no brakes. What do I check?

1 Answer
315

jnebeker75 answered 4 years ago

what you are describing points to a bad power booster. The way we check power boosters is by pumping up the brakes with the key off till the pedal gets stiff, with your foot still holding the brake pedal start the car. the brake pedal should drop a little bit. if its going all the way to the floor the your power booster is losing vacuum.

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