I have a stock 94 silverado 1500 and when i apply the brakes even slightly i can hear what sounds like air escaping from somewhere by the brake pedal. The brakes are harder to push but still work just no power assist. The parking brake light stays on all the time now too. If i turn the truck off ill still get the 1 last power brake then hard again. Also if im sitting at a stoplight holding the brake the engine will go from roughly 650 rpm (idle in drive) to around 500 and after a while the engine light will come on. Im stumped neighbor thinks its the master cyl and a friend thinks its a valve or something. Help appreciated.

5.7l tbi
Automatic
reg cab long bed

 

sounds like you have a bad brake booster......the "air" you hear is a vaccum leak......time to replace the brake booster