I'm trying to isolate a vacuum leak by connecting a vacuum handpump to various systems. The GM book does not say hiw much vacuum these systems should hold. Does anyone know how much vacuum the heater system should be able to hold? I connected the pump to the vac line that connects to the rear of the intake manifold, just before the 't' or check valve that routs the hoses to the vac canister and to the heater control inside the car. I can only get about 4" hg out of it. Is that normal? The heater control is set to 'off', so no air is bleeding thru the control. This is an '89 305 TBI Firebird. Also, what vac should the brake booster be able to hold? How about the carbon can?
Thanx

 

just a WAG but if you're having problems with the heat controls take a look at the "tree" and all the hoses that go into it ,on the back side of the dial on the heater control panel

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ICON Motorsports

1st & 3rd

 

The heater system is OK, I'm just checking everything I can because I have an erratic idle. I already know my brake booster is bad, because it only holds about 5"hg, and it should be able to hold somewhere between 15 and 21 " hg. I can drive the car w/o power brakes and block the vac line feeding the booster, but there still is a leak somewhere else. Thats why I was wondering whether or not the vac system supplying the heater/ac has some type of pressure regulating valve built in to the check valve at the 't'. And as long as I'm at it, I was going to check all the other vac systems, like the carbon can and air cleaner gate.