Braking Issues

A 'bad' check valve would be one that leaks. The check valve is just a one way valve that holds engine vacumn in the booster for 1-2 brake applications. It does not affect the brake system at all. All it can do is make the system feel harder to operate due to no vacumn in the system after turning off the engine. ITS A SAFETY DEVICE ONLY GUYS!
A leaking check valve cannot affect the normal performance of the brake system.




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I meant the other valve(s), which let the booster apply boost when you push the pedal, and which let the pedal return when you let off the pedal. Parts 9 & 17 in this example.



http://auto.howstuffworks.com/auto-parts/brakes/brake-types/power-brake2.htm

 

This is a badass cutaway. I'm still a little lost at how that seal failing gives you a hard brake pedal. Shouldn't that just give you the feeling of no power brakes?

 

The amount of leverage the pedal has is set up with the power assist in mind. So if you don't have any assist, the pedal will feel really hard (to my mind, anyway).

From the link I posted below the pic:

When you hit the brake pedal, the rod cracks open a valve, allowing air to enter the booster on one side of the diaphragm while sealing off the vacuum. This increases pressure on that side of the diaphragm so that it helps to push the rod, which in turn pushes the piston in the master cylinder.
As the brake pedal is released, the valve seals off the outside air supply while reopening the vacuum valve. This restores vacuum to both sides of the diaphragm, allowing everything to return to its original position.

 

Post #1 didn't bleed or post # 20 didn't bleed?

 

So if I'm understanding this correctly, if that valve on rod is bad, the hard pedal feeling is from trying to fight atmospheric pressure trapped inside the body of the booster?

 

I think there are a couple ways you can have a hard pedal.

One is if the diaphragm is torn, in which case the engine can't develop a vacuum against the diaphragm. That leaves you trying to push hydraulic fluid with your own strength. But I think the pedal should return fine in that case.

The other way is if the valve is stuck. That leaves vacuum on both sides of the diaphragm, instead of vacuum on one side and atmospheric pressure on the other. Again, that leaves you pushing the hydraulic fluid with your own strength. But once you get it pushed down, that valve is supposed to move and admit vacuum to that side of the diaphragm, which would balance out the vacuum from the other side and let the spring return the pedal to a relaxed position. If it can't move and let vacuum back in, the pedal would not return well.