Vacuum drops, engine stalls when I step on the brakes

[valuecom]

Sent: 6/19/2003

BASIC INFO:
White plate: Winnebago MFG 11/71 ID# 3215011112546 Model: C20
Metal Plate: GMC CPE372J107250
Looks to be a 1972, Chevrolet, 1 ton, forward control, motorhome chassis built in Janesville, WI., the 7250th such unit off the line.

I have a vacuum gauge on my 72 Brave. When I first start the engine and I depress the brakes the vacuum drops and stalls the engine. No visible booster at master cylinder. Although there is a large, maybe 1" hose running back from intake. This attaches to two boosters
inside the frame about half way back. After about 5 mins the vacuum pressure must build up because now when I hit the brakes it doesn't stall but the vacuum gauge still takes a dive. I climbed under it and listened when my son depressed the brakes. I did make a Humm sound but it didn't hiss as I would expect a leak to do. Is there some way to trouble shoot this? If one goes bad does it effect the performance of both?

Other info:
Front brakes : Disk
Back Brakes: Drum
Master Cylinder: single on firewall.
Parking Brake: Manual and working.

Thanks for your help in advance!!
Roger Bredow

[denisondc]

Sent: 6/19/2003

I don't know much about the GMC chassis, but something is wrong if the engine dies when you stomp on the pedal, even right after starting. I would say you have a constant vacuum leak, which is making the engine run too lean - it may not be the brake boosters though - could be a maladjusted choke, a leaking carb base gasket, cracked hose,  anything. When warmed up it would be less likely to stall anyway. Yes, the vacuum gauge would show that you had pushed the brakes, but it should show about as much drop when you release the brakes as when you push the pedal down, I think. At least the vacuum should not be any lower while you are holding the pedal down. There shouldn't be any sound from your booster once the pedal is down either - it may also have a valve leaking a little, or small leak in a bladder. The classic way to tell if your boosters are working okay, is to judge the braking pressure required to stop the RV from low speed, with the engine running, and also at low speed; with the engine shut off and the vacuum in the boosters depleted. If you have one booster with a leak, it is probably going to impact the other booster, but in proportion to whether the leak is bad enough to lower the effective vacuum in the booster itself. If you shut the engine off and wait two minutes, you should still have good stopping power, due to the vacuum still in the boosters, and the one way valves. If not - something wrong. And you might have two or three leak problems to hunt down. My dodge chassis boosters hold most of that vacuum for at least five minutes after the engine is shut off, if I don't push the pedal. The boosters are also located half way back, inside the frame rails - no booster on the master cylinder. Please let us know what you find. denison

[valuecom]

Sent: 6/20/2003

UPDATE:
1. Found a company with chassis owners/ maintenance manuals going back to 30's called
FAXON 1-800-458-2734
mine was only 14.95 including postage. Haven't received it yet. More on this coming

2. (refer to first post)
I finally found someone is this small town that had actually worked on one of these long ago. He told me to get under the Winne with the motor running and someone at the wheel. Then unhook one of the intake hoses.
As soon as you do this the vacuum drops so put your finger over the hose to the intake manifold (where the vacuum is being created). Repeat with both boosters. I tried this, but became baffled when depressing the brakes
the booster I had disconnected stated sucking air when the brakes where depressed. I clarify this by saying, I had the pipe T plugged coming from the engine. But the hose coming from the disconnected booster started sucking air. I plugged that and the engine began running normally again, thus the vacuum was back to where it was before the test. (hope you understand this). I deduct from this the second booster may be running in parallel
or at least has some sort of equalization going on.

The good thing I noticed was the engine wasn't killing down so I know the problem was at least beyond the T where the two booster lines. It didn't eliminate a possible bad booster but it reduced the possiblity of a bad line
from the manifold to the "T".

I was about to take the line off the second booster when I noticed another 1/2" line going back towards the engine compartment. Followed it back. On the end of this is was a cylindrical shaped filter next to the passenger side exaust manifold. So I removed it, brakes not depressed. Depressed the brakes, vacuum dropped again. Plugged it with finger, (I love the engine being right next to me!!), the vacuum when back up farther than it did with the filter on.

So a did the following backyard mechanic test: I plugged the end of the tube that would normally hold the
filter with a wad of duct tape and took it out on the street. It seemed to stop about %50 quicker with it plugged as opposed to with the filter thing. It still wasn't braking the way I remember but at least I think I found part of the problem. I seem to remember seeing one of these filter gizmos somewhere before. I will need to get a new one.
Then I will retest. I sure wish I knew what the filter was for?? Why would there be an open end , filtered or not on a vacuum system.

Conclusion:
I now believe the filter is at least partially bad. I suspect I have a failing booster or one with a small leak. The fact the boosters seem to be partially working by the noticeable change in braking distance with the system close tells me thay are at least working some what at least or that they are working but I have leaks elsewhere.

PS. I know I have exhaust manifold leaks in gaskets, could this effect it?

I had to go to Little League with the kids so I ever finished the second half of the test of removing the line from the second booster. Will do this this weekend. I just wanted to get this down to help or confuse further anyone with a GMC chassis.

More later.......... Roger

[denisondc]

Sent: 6/20/2003

Good investigative work! Some of your findings would apply to any truck with the similar dual remote boosters - which is probably mostly bendix components. The filter thing you found is the atmospheric air intake to the boosters. The way the boosters work involves a vacuum being drawn on both sides of the bladder, and the braking pressure opens a valve to admit atmospheric air to the back side of the bladder, which makes it operate the booster cylinder, and provide the assisted braking effect - and the atmospheric air comes in via that filter. When working right you should hear a soft whoosh from the filter when the engine is running and you stomp on the brake pedal.

Your exhaust manifold/system leaks would have no effect on the brakes or the boosters.  And as far as I know, each boosters vacuum source is connected in parallel with the other, so a leak in one bladder would also jeopardize the vacuum level in the other booster.

denison