When a brake vacuum booster fails?

[canada karl]

Backed the 77 530i out of the driveway and only had about half pedal resistance to get braking and had to press fairly hard to stop. Previously only about an inch down on the pedal to get good braking. Does this sound like a brake vacuum booster problem. I do not see any external leaks in the system. What should I be looking for and testing? Karl

[PatinaBeforePolish]

If the brake pedal needs more travel to engage the brakes, you have a leak somewhere. If you have to push hard for the same results then it's vacuum, master cylinder or caliper related. From your description, it sounds like some component randomly failed. Is the fluid level down? I'd start by checking the one way valve in the vacuum line and verifying the wheels turn freely. Then look for leaks anywhere brake fluid travels including inside and behind the carpet in the driver's footwell. How old or crusty is the master cylinder and how often is the fluid changed? The seals could have failed.

[canada karl]

The calipers and MC are fairly new but I will check for leaks. i think you might be on to something with that one way valve. What symptoms if it fails? I will check that out as I think I have a spare. The brake boosters on these cars often fill with fluid....is that motor oil being sucked in? The brake resevoir does not appear to be low on brake fluid?

[HB528i]

When the MC leaks brake fluid fills the booster, it isn’t engine oil.

[canada karl]

Strange....the resevoir isn't low on brake fluid?

[PatinaBeforePolish]

When I replaced the master cylinder on my 79, there was about an ounce of watery oil in the canister. My theory was that the canister held vacuum after the car was shut off and pulled oil fumes through a faulty check valve which, over time, accumulated as watery oil. But my brakes worked as intended and didn’t have the symptoms you’re describing. I suppose if the check valve were installed backwards pedal pressure would be affected but I never tested it that way. The valves are readily available and cheap enough from the aftermarket or BMW and it wouldn’t hurt to replace it anyway. You can blow through it to determine the correct direction. I believe the white side is on the vacuum booster side.