The brake pedal has two switches....and electronic one for the Brake Lights and Cruise Control Electronics shutoff...and a second that is a pressure vent switch, this is used to vent vacuum pressure from the Cruise Control system....
The Cruise Control System uses a vacuum pump to pull a rubber diaphram bellows connected to the throttle linkage, when you set a speed, the pump pulls a vacuum and sucks the bellows to pull the throtle to the set speed.
When you brake, you have to one shut the brains off, and two vent this holding vacuum pressue, hence the tiny hiss....
If it is quite a loud noise or a more agressive sounding hiss, it might be something else.
The P38 uses powered brakes and not a vacuum drum of old.....
When you press the pedal, this pulls on a linkage inside what would be called the Master cylinder this has the effect of opening valve ports to direct hydraulic pressure to the calipers to apply the brakes, when you release the brakes, you are colsing these valves and opening the vent side to vent the hydraulic pressure back to the brake fluid tank....this hiss or whoosh is the brake fluid being reurned through the 'master cylinder'
Is it more of a hiss or whoosh?? both should be quite faintish...a loud noise would require further investigation....