Air leak between Master cyl and power booster

[ThurzNite]

I have a problem similar to this guy: Soft brake pedal when car is on--booseter issue?

When car is running, the pedal's mushy. Constant pedal pressure doesn't drop the pedal to the floor. I'm not leaking brake fluid. I don't think it's the brake lines. Lines have been bled multiple times.

I do hear a vacuum leak. It gets pretty loud when then engine is on and I press the brakes. The leak is at the bottom of where the master cylinder and brake power booster meets. In fact, with the brake pedal depressed, I can feel air coming out of the junction.

With the engine on, brake pedal depressed, I can turn off the engine and the pedal pushes up at me and gets rock hard. With other cars, this doesn't happen. In fact, on other cars, I can usually get another pump on the pedal, but my car doesn't.

I'm certain it's a vacuum leak, but I'm not sure if I need to replace the master cyl or the brake booster. Any ideas on how to diagnose this? I can't go a day without the car, so can't have down time to order necessary parts, nor do I want to spend $$$ on buying both.

Thanks!

Dr J

[AlmightyCamry777]

Disconnect the vacuum line to the brake booster and plug it. Run the engine and see now if the brake pedal feeling changes. If it doesn't, maybe you need a new booster.

[73sport]

Where the master cylinder mounts there is a pushrod/seal assembly separating the engine vacuum from MC from the pushrod inside the booster (connects to brake pedal). The seal is replaceable. Remove the master cylinder and inspect the seal. Its very like the master cylinder is or was leaking and rust formed on the pushrod which later took out the seal making the leak you hear.

The seal must have the proper lubrication and the pushrod shaft cannot be rusty, pitted, burs or other defects that will harm the seal. So you may need a new pushrod.

[reluctantmechanic]

Originally Posted by 73sport

Where the master cylinder mounts there is a pushrod/seal assembly separating the engine vacuum from MC from the pushrod inside the booster (connects to brake pedal). The seal is replaceable. Remove the master cylinder and inspect the seal. Its very like the master cylinder is or was leaking and rust formed on the pushrod which later took out the seal making the leak you hear.

The seal must have the proper lubrication and the pushrod shaft cannot be rusty, pitted, burs or other defects that will harm the seal. So you may need a new pushrod.

I'm being quoted 700bucks to replace the brake booster, and would very much like to just replace the seal - can I just buy one from toyota? and about how much does this seal cost?

Thanks