Brake booster check valve.
 

My car has always idled horribly, even worse than my old car, which had twin SU-carbs. That's hard to do. So I'm in the process of patching up my engine's many, many vacuum leaks. One of which I think is the brake booster check valve, if that's broken or leaking can that cause a hissing sound that goes away when you press on the brakes? The brake pedal is also a little stiffer than it should be. I never thought it was stiff until I compared it to that of my roommate's Volvo 240, which is in much better condition. Well, not probably by you guys' standards, but much better than mine. I've got three friends that won't even ride in mine anymore since the driveshaft exploded that one night. But I digress.

Also, where is the brake booster check valve? The hissing sound sounds like it's coming from inside the car, though it's a little hard to hear anything outside the car whenever the engine is running. I guess the brake booster reservoir might be leaking too.

http://www.fcpgroton.com/product-exe...category_id/57

It's pressed into the booster where the vacuum line attaches.

I'm thinking that if you're hearing a hissing leak sound INside the car that there's a leak in your HVAC control system. Plug up the vac line from the intake mani to the fire wall to see if the sound goes away (courtesy of Bubba240 in one of my threads)

Oh yeah I forgot about those. Lord knows why those are vacuum powered, though it's funny listening to my vents squeak back open when I take my foot off the gas after climbing a hill.

About the brake booster though, it also idles down when I press on the brake, and it won't start if my foot's on the brake.