I just replaced the MC/Booster on an 85 Iroc with one from a 1990 I found on Ebay, is it normal to hear that air whooshing noise when pressing down on the brake pedal?

 

it will make some noise, yes

 

seams loud from what i remember of this vintage

 

The packing may be missing from the area where the pedal rod enters the booster. This is both an air filter and silencer. I've been known to reduce the amount of packing for a quicker brake response.

RBob.

 

Bob, what should I expect to see (looking from under dash) for packing? Is it visible?

 

I'm not sure what it is on our 3rd gens. The older cars GM used a jute packing, same material that is used under the carpets.

Although, the '92 FSM shows a plastic or rubber boot. Then further into the booster the silencer, which looks like a big jute or felt washer in the diagram.

RBob.