1977 Porsche 911 Brake booster/Master cylinder

2 things can cause this problem a leaky vacume hose leading to the botom drivers side area of the booster just below the master cyl. But not likely '' most likely'' its the return spring down at the base of pedal its a coil spring with a hook on one end that wraps around the pedal to hold it in the up'' position take a peek at it '' good luck. Hope this helps. Mechtrix