The valve that controls the vacuum for my heater/air conditioner is stuck part way open. It hisses all the time the engine is running and it allows the airbrakes to drain down to about 40 pounds. I noticed a cylinder on the valve assembly that's held on with four screws. Can I remove that and clean or lubricate it?

This sounds like 2 problems in your venturi vacuum generator...

The item that you are talking about taking apart is the low psi cutoff and that should be set to around 80-90lbs.

Chances are, you have a vacuum leak and the generator is compensating by running all of the time.

This device generates vacuum for your dash a/c switches and door flaps. Try turning off the dash heater and a/c switches to see if it stops.

read this post too

is this the item Richard?

The dash vacuum should not have any connection to the brake system. You might have an isolation valve problem that allows the brake air reservoir to leak past the valve, or leaks in the brake system. Not having dash vacuum is much different from losing brake high pressure air and the 65 to 70 psi safety valve that protects the brake system is, in pilot speak, "A downing gripe!". As Bill suggests you may have two problems.

Richard, I had a similar problem with my 91 Breakaway this post Volvo by MWrench solved my problem. This gives you an independent source of air pressure for the dash air/heater control. Also can replace the one in the rear compartment for the cruise control the same way. Talked with the gentleman who purchased my Breakaway a few weeks ago and he said that it was still working great. I purchased the Volvo pump on Ebay...about $40

Bill,

That looks like it except mine is connected to a block with two air lines, one coming from the back of the coach and the other going up toward the dashboard. When I clamped the two rubber hoses on the device, the amount of air slowed down but did not stop. It does stop hissing when the pressure hits about 70 pounds on the front brakes.

OK, it appears that the regulator is working correctly if it stops hissing at about 70 PSI. That certainly points to a vacuum leak or a defective venturi vacuum generator.

there should be 2 hoses connected to the vacuum venturi generator, one if from the air supply and the other is the vacuum source that would go to the small vacuum reservoirs. If you pinch off the hose going to the vacuum reservoir, and the hissing stops. then you have a vacuum leak in the HVAC system. probably a deteriorated hose or hose leak at a connection.

If it does not stop hissing, then the venturi generator is defective and yo will have to go the route I did with a replacement vacuum pump.

If you pinch off the air supply hose it will probably blow off the hose. Also if you try to block the port of the vacuum generator where the air is coming out, it definitely will blow of the hose from the air supply to the venturi generator.

If you want to talk, you may call me at:

408.354.0123

Well I took out my venturi vacuum generator and replaced with an air chuck. Now I just have to get a vacuum pump to replace so I have heat and defrost. It gets cold up here in Minnesnowta. Air conditioner after all I read it abandoned. I had found one spot where it the hose had been compromised. I also have to replace cable because it has buckled on each end even thou it is free. Can't get it to stay straight. Hope to replace just the internal wire.