1995 chevy gmt 400.
Brake problem
Pressing on brake pedal hear air hiss (under dash)? and idle goes up. Pedal is hard to push and brakes dont seem apply unless you push hard.Checked the check valve and engine vacuum. Any advice? Ty

 

My take is brake vacuum booster

 

Ok. Ty for taking the time to respond. I appreciate it.

 

Sounds like the daiphram in the booster is leaking. the rise in RPM is due to air leaking into the manifold making the fuel mixture lean which causes the engine to rev up.