Hissing brake booster?

[snowballfisher]

I am working on a 1968 camaro w/power brakes.  I've just recently replaced all the rubber brakes hoses front and back, new pads/calipers and shoes/wheel cylinders.  I bled the system and the brakes work but not very well at all.  When I hit the brakes there's a hissing sound coming from the booster and I have a hard pedal but weak brakes.  It seems like my rpm's climb when I hit the brakes also.  Anyone have any idea if this is a bad booster or just a check valve?  The master cylinder and the booster are about the only things I haven't replaced, but I cleaned the master cylinder pretty well and flushed it out.

[drdave69]

Change the check valve first and if that doesn't fix the leak you may need to have the booster rebuilt. Get someone to help you figure out exactly where the air is escaping.

[snowballfisher]

Thanks... I'll do just that.

[Gramps69Z]

If you start the car, and then look under the dash where the pedal rod goes into the booster.  Push on the brake pedal and listen for the leak. That is normally where it is.

Fireman John

[JohnZ]

There's a foam seal where the pedal pushrod enters the back of the booster - its purpose is to muffle the sound of air entering the rear chamber of the booster when you step on the brakes (which is how the booster works); if that foam seal is gone or deteriorated, you'll hear a "whoosh" sound under the dash when you step on the pedal. 

[snowballfisher]

Thanks for all the answers everyone.  I did break down and replace this hissing booster and it solved my problem.  The brakes also feel about ten times more, "there."  This car had an engine fire long ago and I think it probably trashed that internal diaphragm.  Anyway, about $1000.00 later and all new brakes from top to bottom and front to back, I have a great pedal and reliable stopping power.  Next project... um suspension 

[lakeholme]

And like my daddy always told me, "Before you make her go faster, first make sure she'll stop!"