Whenever I press brake It don’t rev its only when I let go of brake it revs less then half. Any ideas

 

Do you have a one-way check valve between your brake booster and the intake manifold ???

 

A slight rev increase then back down is normal. This is how the brake booster works, it lets air into the engine to build up a a vacuum for the next stop. Ordinarily people don't notice it since releasing the brakes is usually followed by hitting the gas anyway.

If the revs go real high and stay up, the booster may be leaky. Check by disconnecting and blocking off the vacuum hose.