My Ford F250 4x4 brake pedal goes to the floor when you start the truck and stay's to the floor until you shut the engine off then it returns? The brake booster, master cyl, rotors, pads, brake lines, drums, brake shoes, and brake cylinders have all been replaced.

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    Is the pedal firm when the truck isn't running? Was the master cylinder bench-bled before installing, and are you sure everything was bled properly once installed (without more air entering the master)? – JPhi1618 Nov 9 '15 at 19:08
  • the pedal goes to the floor when the truck starts up. the pedal is firm when the truck is off. – bruce Nov 9 '15 at 19:21
  • Agree with @JPhi1618 line of thinking ... I'd bet the system needs to be properly bled. – Pᴀᴜʟsᴛᴇʀ2 Nov 9 '15 at 19:47
  • Is it a diesel or gas? Does the brake booster run off a vacuum pump? – mikes Nov 9 '15 at 21:55
  • Is there a way to get the booster valve that is stuck open free? – bruce Nov 10 '15 at 16:32

One of the valves in the brake servo (booster) is stuck open. When you start the engine the vacuum produced by the engine (or a separate vacuum pump) is used to assist braking pressure from your foot. As you press the brake pedal, a valve in the servo opens to allow the vacuum to pull against the diaphragm in the servo to assist you. If a valve is stuck open it will keep sucking until the pedal goes to the floor and will only release once the engine has stopped.

Presumably it is putting the brakes on when this happens. If this is true, it is nothing to do with bleeding.

Depending on the design of the servo, there may be a pipe that connects the servo casing to the servo valve block, if this is leaking or missing, the same symptoms would occur. Any air leak into the casing in the rear of the servo body will also produce these symptoms.

  • That's interesting. Where is this Brake Servo located? Is it a integrated part of the booster? – JPhi1618 Nov 9 '15 at 19:56
  • Sorry, you probably call it a booster, it is known as a servo over here, I will update my answer. – HandyHowie Nov 9 '15 at 19:58
  • I hadn't thought of this but it is a very viable answer ... probably quite accurate as well. – Pᴀᴜʟsᴛᴇʀ2 Nov 9 '15 at 22:32
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    It would be very difficult trying to explain something like this without having the exact unit available to me. It is probably also not something I would recommend trying to repair. Did you install the new booster? Have you checked it is plumbed in correctly? Can you check any pipes on it for leaks? – HandyHowie Nov 10 '15 at 16:53
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    The air will be getting sucked in, rather than leaking out. I am pretty sure if you can find where the hissing noise is coming from, you will find your problem. – HandyHowie Nov 11 '15 at 14:25

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