brake woes

[ryanruggles at uniserve.com (Ryan Ruggles)]

I've got an 84 Gti.  Last night I lost my brake pressure.  There are no
leaks in the system.  The brake master cylinder is not leaking on the peddle
side either.  So did my master cylinder just bite the big one.  I think it's
maybe leaking internally.  The funny thing is that the brake peddle feels
fine and builds pressure when the car is not running.. As soon as I turn on
the engine, the peddle goes to the floor and I have to pump it up just to
get the brakes to grab a bit.  Vacuum problem?  Any ideas?   I don't want to
buy a master cylinder if I don't need one.

Cheers

Ryan