Diagnose this: 85 Brat, power brakes. Not brake pedal either with vehicle off or with engine running UNLESS I get the engine RPM's up around 3000 at which time I get a firm pedal. No evidence of leaks in brake lining, calipers, wheel cylinders or at master cylinder or booster. Vacuum hose LOOKS good from manifold to booster. There is a small metal check valve at the midpoint of the manifold-to-booster hose.

 

What's the culprit? Just trying to save time in troubleshooting.

Make sure the check valve is going the right way, the arrow should be pointed towards the intake IIRC I don't know about subaru power brakes but in other cars a bad booster would tend to make your brakes harder to push not sloppier, it sounds to me like the MC is failing but it doesn't explain why it starts working when your Rpms come up, that makes it seem like a bad booster.

I was thinking booster myself. Usually when a MC goes it works or it doesn't. Anyone else?

I was thinking booster myself. Usually when a MC goes it works or it doesn't. Anyone else?

If the MC is starting to fail the booster might be putting too much pressure on the master cylinder, as you rev the engine up the vaccume of the motor decreases and makes the booster work less making the pedal firmer.

How long this been happening?

Have you bled the system?

Your MC could be leaking internally.

If the brake pedal feels spongy, mushy, or has to be pumped a few times before any resistance is felt, air is in the system.

If brake pedal feels solid, but goes to within 3 inches of the floor or so before any resistance is felt, brake pads/shoes aren't making contact as quickly as they should and probably need adjusting. 

 

This thread reminds me of an episode of House.

Edited May 11, 201312 yr by jonas

Zero pedal until the engine RPM's are up.