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My 2007 Explorer has recently started lifting the brake pedal (with considerable force) almost immediately after I turn it off. Why? Could this be a vacuum leak in the booster? The brakes work perfectly while the car is running.

Update: From the responses, it seems I didn't explain myself well, so I'll try a slightly different angle/format:

  • as I drive the car around, the brakes and everything else behave correctly
  • when I come to a stop, holding the brake pedal down, I turn the key to off (still holding the brake pedal)
  • without changing the amount of force on the pedal, the pedal forces itself up against my foot
  • More than likely it is the ABS system doing this. – Moab Jul 2 '17 at 1:33
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My van is doing the same samething. Found it to be the brake booster check valve not holding vacuum sometimes.

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A vacuum leak in the booster would lead to hard brakes, longer stopping distances and engine stalls when the brakes are applied. If there was low vacuum in the system you'd notice other symptoms.

What it sounds like you have is a worn vacuum booster or master cylinder, or both. If the master cylinder wears down too much the booster can travel forward and get stuck, it's also possible that the springs in the master cylinder don't have the power to push the booster pushrod back. The booster has a spring in it which pushes the plate back up when the brake is released, it's also possible that has worn out, or that corrosion/wear in the assembly is causing it to hang.

If I had to choose one or the other I'd probably replace the booster first, although if the master cylinder is old too it might be better to replace both as problems in one could damage the other.

The booster spring has to be strong enough to push the pedal back against the power of the vacuum, if the spring gets too weak it might not have enough power to fight that force. When you turn off the engine the vacuum force disappears and the spring can push the pedal back. It could also be one of the internal valves not working properly.

A good article on the operation of brake boosters states:

A broken spring most often causes the brake pedal to not return fully when released.

And

Releasing the brake pedal allow the internal spring to push the diaphragm and push rod back and operates the two-way valve. This valve blocks atmospheric pressure to the rear chamber. Simultaneously, it opens the chamber to vacuum. This evacuates the rear chamber and assist in returning the booster to a state of rest.

  • Wouldn't a worn master cylinder be apparent while the car is running? Either seepage past the seals (sinking pedal) or a return spring (pedal slow to return)? If the booster return spring were weak, I'd expect the booster to suck a lot of vacuum and the engine to run poorly. Nor would it explain the lifting pedal. – user7264855 Jun 30 '17 at 12:04
  • Most wear is gradual, you get used to small changes in driving such that you may not realize. A worn master cylinder may show by leaking, or it may not. The booster will not suck enough vacuum to impact the engine if the membrane is torn. See my edit for the pedal lift. – GdD Jun 30 '17 at 12:13
  • Your assertion that the booster spring has to overcome the force of the vacuum doesn't sound right to me. If the spring were strong enough to overcome the vacuum, the pedal would be rather hard to press. When the engine is turned off, the vacuum should not disappear in the booster if the input check valve is working. Hmm. Ok, that's my latest guess. Perhaps the input check valve no longer seals properly and the vacuum leaks from the booster. Which means the 'boost' would dissipate, forcing the pedal back up. I'll see if I can verify that. – user7264855 Jun 30 '17 at 12:43
  • The spring's strength is gauged to balance out the force of the vacuum. I'm not saying 100% it's the spring, but I'm saying it's most likely something in the booster. You might want to do a vacuum test too in case you're getting too much. – GdD Jun 30 '17 at 13:25
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Sounded like they said. When the truck was turned off. If so there would be no vacuum to help with breaking do you drive often with the engine off. Sounds like you save a lot of gas

  • Bet you're power steering doesn't work to ease when the key is off either – Danny Jul 3 '17 at 14:30
  • I'm not following your answer, @Danny. Who's comment/answer are you referring to? I'm not trying to drive with the engine off. Maybe my updated question is clearer? Thanks. – user7264855 Jul 4 '17 at 22:28
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It is the vacuum check valve for the vacuum hose from the manifold into the booster. When operating correctly, it should only allow air flow from the booster to the manifold in order to create a vacuum on the piston/hydraulic side of the booster. In this case, the interior valve has disintegrated and allows air flow in either direction.

Vacuum Check Valve

In this vehicle, the check valve is extremely easy to remove and test. Locating the hose from the manifold to the booster, the check valve is the plastic 90 degree connector into the booster; there are no clamps so simply pull the check valve from the rubber grommet in the booster and then pull the valve from the hose.

Air should only flow from the booster side to the manifold, so blow into the manifold side to test. If air flows in this direction, it is not working. This was a nine dollar part from NAPA for me.

So why does the pedal lift? The manifold is maintaining vacuum while the engine is running, but as soon as the engine stops, air flows back into the vacuum side of the booster and I loose all the assistance I was using to keep the pedal at that level. Unless I apply a great deal more force, the pressure on the brake hydraulics will overcome the pressure from my foot, raising the pedal.

This check valve provides a degree of safety for the case in which the engine stalls while driving. When the valve is working, vacuum is maintained even after the engine is shut off, allowing for at least one vacuum assisted stop of the vehicle. After the vacuum is gone, the driver will need to apply far more force to engage the brake effectively.

Most explanations of vacuum brake booster do a lot of hand waving. The only complete description that I could find of the valving and how equilibrium is achieved was in this youtube video. Once I saw this, it seemed there could only be this one (single) reason.

Thanks to @GdD for helping me think this through.

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