Checked the check valve and it's ok. Brake booster definitely needs replacing, I've come to the point where I shut off the engine with the pedal pressed and as soon as the engine is off, the pedal feels much stiffer and if you can hear a hissing. Is it dangerous to drive like that? Just bled the brakes due to a master cylinder upgrade and I don't want to do it again. Is it possible to change the booster without unbolting the brake lines from the MC? I highly doubt it though. Thanks!