I finally got the message that I needed to replace the brake booster after the tell-tale hissing under the dash coincided with a supreme lack of brakes! Sort of liked the 'air brakes' hissing, but I could not deal with 'NO BRAKES!!' After swapping that thing out for $80, I noticed the oddball erratic idle at startup had disappeared. WOW! It was'nt the IAC valve after all. It has been the vacuum booster all this time! The tranny began shifting differently and low speed (rpms) acceleration was greatly enhanced. What a difference a brake booster makes. I wish it would have gone a long time ago. How do you check a brake booster if it does not totally fail?

 

You still had brakes, you just didn't have boost. There are federal requirements for vehicle performance in a no-boost condition.

It may have felt like no brakes, but thats just because you have to push really hard on the brake pedal.

If you are developing a leak you will notice a hard pedal when you get in the truck (and the engine is off). Your booster should hold vacuum for at least 24 hours after you shut the engine off.

If the pedal is hard when you get in the truck (and its been less than a day since you drove last) you should first get a new check valve (the thing the vacuum hose attaches to). If that doesn't fix it, there is a good chance you have a leak around the valve body and you will want to replace the booster.

The other thing to do is shut off your radio and AC once in awhile and listen. If you hear air noise around the brake pedal you should replace your booster.