Possible master cylinder? or maybe nothing at all?

[crm103]

Ok, so I'm am fairly new to E30's, been an aircooled VW guy my entire life. So bear with me. I noticed today when stopped at a light, my brake pedal would very slowly fade. When I came to a stop, the pressure was fine for about 10 seconds, then it would fade a bit. Stop for a few more seconds, then fade a bit more etc. It would never reach the floor (maybe halfway if that after 30seconds of holding it). And when i really stomped on it, it was the same thing and slowly fading at the same rate. It wasn't a constant fade. When the car is off, the pedal stays hard as a rock. And the car still tries to rip my face off when i stomp on them while moving with no fade. I checked for leaks, none were found. Brake fluid was recently flushed w/ new pads and rotors. Maybe its nothing and I'm just trying to find something wrong, or maybe signs of a master cylinder going bad? Tried searching and couldn't find anything like this. Thanks in advance for any help!

oh..btw its an '88 325i

[jlevie]

The sinking brake pedal is a loss of fluid in the brake circuit. If you can't find any leaks from under the car, then the master cylinder is bad. A bad master cylinder can leak fluid back to the reservoir, or it can leak into the brake booster. If the master is leaking into the booster, both the booster and master must be replaced. Brake fluid will kill the booster fairly quickly.

When the engine is running, the booster applies a lot more pressure on the master cylinder than you can with your foot with the engine off. That is why you don't fee the sinking pedal with the engine off.