when you press on brake pedal the idle goes way down and you can hear air

I have a 1985 chevette when you press on brake pedal the idle goes way down and you can hear air. But the air leak is not coming from the brake pedal. It has standard brakes

The brake booster operates on a vacuum created by the engine. There is a thick wall hose connecting the inlet manifold to the booster. The booster has a non return valve in it which allows a couple of pumps of the pedal in the event of the engine stalling. This should allow you to bring the car to a safe stop. You don't say if the brake pedal feels normal but the change in idle is caused by you operating the booster. My guess is that the air noise you can hear is a leak in the hose or the booster itself. This may require the assistance of a good DIY mechanic to determine which it is. Make sure you get the correct thick wall hose if its that, that is faulty, because any normal hose will be sucked flat by the vacuum and you will have no brakes!! Should the hose be OK and the booster be the noise you can hear then replacement is the only option. A quick call to a parts house with the vehicle details will soon tell you. If you think about it your car is 30 years old, pretty good going really.

It looks like the brake power booster is defective. A vacuum problem. You say it has standard breaks. Then the brake pedal lever is somehow interrupting a vacuum hose.

Some part of the brake linkage must be pressing on a vacuum hose. Check under dash as well as under hood.