holding air in a GM 4106

silversport · October 31, 2011, 04:42:32 PM

OK, I am attempting to eliminate air leaks and have the system hold pressure. I have read some of the back topics on air leaks and starting to think I am getting to anal. I am to the point were I have capped all tank opening, having the three tank with the line that goes to the rear brake relay, line to the governor and the engine stop and fast idle solenoid left in the system. The engine air compressor is also in the system but I am using an auxiliary compressor to fill the system, yesterday afternoon I fill the system to 115 psi and to night it is at 50 psi. Is there a possibility that the bus air compressor is letting air feed back or the relay has a leak? I used a quart of soapy water on ever connection I can not find bubbles. Am I missing something, would like to have this part of the system hold air before I start the fun stuff (suspension system)
Robert
GM 4106

Cary and Don · October 31, 2011, 05:16:58 PM

The two big leakers are the brakes and the air suspension. Just walking around inside will cause the air suspension to loose a little air. Parking off level will make it try to level itself, thus loosing a little air. Run a DOT air check on the brakes. I think there are previous posts on the way to do the test. This can tell you where there might be a leak. We usually find these little leaks in the brake system. You might not ever get rid of all the leaks, but you still have some pretty good ones hiding in there now.

Don and Cary
1973 05 Eagle
GM 4107
Neoplan AN340

silversport · October 31, 2011, 05:38:32 PM

but Don, none of those items are even hooked up to the air system.

Utahclaimjumper · October 31, 2011, 06:38:36 PM

If your check valve is working correctly,,you should not have "backfeed" back to the compressor,, check your manual for the check valve on the inlet to the wet tank.>>>Dan ( The check valve can be removed and repaired per the manual)

skihor · October 31, 2011, 07:10:12 PM

It is possible. We have a 4 corner individual air bag valve system. The stock leveling systems are disconnected. In the last 8 years I've had to add less than 5 pounds TOTAL. On the other hand the rest of the system leaks down to 50 lbs in 12 hours and nearly 0 in 24. It passes a D.O.T. brake test so I haven't cared much. I'm a light duty semi truck mechanic and there are so many valves/lines/devices etc... one can go nuts trying to achieve an air system without leaks of some kind. Even replacing everything it's hit and miss at best.
One of our trucks still had 70 lbs after setting for 2 months yet wouldn't pass a D.O.T brake test. Go figure.
So armed with soap water and a partner check anywhere that air goes in or out. Drain valves and brake cans are a great place to start. Plug each can one at a time and wait overnite, see what happens. Seperate front and rear brake systems also with that. Drain valves that have been exposed to water for the last 40+ years are suspect as well.

Don & Sheila

Flatspot · October 31, 2011, 07:29:40 PM

Had one that ran me nuts until I found the Air Horn valve was leaking by.
Windshield wiper control valves are known to leak also.

Happy Trails

RJ · October 31, 2011, 07:29:55 PM

Robert -

Before you drive yourself crazy, I can tell you that at the transit property I worked for, brand new buses, both from Flxible and Gillig, would often be sitting cattywhompus in the morning after the air leaked out of the suspension system overnight.

FWIW & HTH. . .

Flatspot · October 31, 2011, 07:39:54 PM

OK
Now that I reread your initial post. Loosten or pull the discharge hose off the (bus) air compressor outlet and see if you have air leaking past the discharge check valve. With it connected to the compressor you won't hear it. That is what I'd try next

Happy Trails

desi arnaz · October 31, 2011, 09:45:49 PM

I found my leak aht the push,pull brake knob on the left side of the dash near. Your elbow. It leaks like a sive...

bevans6 · November 01, 2011, 06:38:22 AM

If you are doing this anyway, I would cap the line feeding the wet tank, from the air dryer or the discharge line from the compressor. Just get the tanks in the system. Cap the signal line going to the governor. Add a schrader valve somewhere to fill.

Brian

silversport · November 01, 2011, 04:46:27 PM

Looks like the air compressor governor needs rebuilding( I will do that later) , hooked up the horn,gauge,air throttle, & wipers, success. Next came the parking brake valve, it leaks, I am not sure, since I haven't a manual for the mini-max brakes or this valve what I am looking for. I have taken the valve apart, cleaned it and tried again with no luck. The spring may be tired or the rubber disk could be gone. Wondering if there is a rebuild kit for this valve. The valve handle is yellow says pull to apply parking brake push to release, there are number on the body with a diamond with MG on the inside on diamond. Something I just though of dose this valve need to be hooked up to the brake relay to work properly? did no try that.
Robert

RJ · November 02, 2011, 12:02:38 AM

Robert -

Quote from: silversport on November 01, 2011, 04:46:27 PM
Looks like the air compressor governor needs rebuilding( I will do that later)

Governors are cheap, less than $30 most places. Just take your old one into the HD truck parts place and ask for one just like it. Not worth the effort to rebuild.

Quote from: silversport on November 01, 2011, 04:46:27 PM
The valve handle is yellow says pull to apply parking brake push to release, there are number on the body with a diamond with MG on the inside on diamond.

These are very common, have been used in nearly all air-brake equipped vehicles for over 30 years. Same as the engine governor, just take your old one in at the same time to get a correct replacement. Again, not worth rebuilding.

FWIW & HTH. . .