Help with air system

[bluegrassmidtenn]

Hello all,
I would like to ask help from the bus gurus out here:

I purchased a 1975 MCI mc8 crusader pretty cheap. Non running. Got the 8v-71 back to putting again. Bus has been sitting for about 2 years. Once I got it running, the air won't go past about 15 pounds on the gauge. Found a hole in the 1/4" copper drain line at the engine compartment. Fixed that. Still won't go past 15lbs. I'm new to this, and these air systems confuse me.
How can I get this thing to go to 100psi so I can get it off this man's property? Any help would be appreciated very much.
Ed

[Nova Eona]

It can help to think of the air system like a big tree; air comes in the trunk, and feeds all the systems at the ends of the branches. If any branch has a leak, the whole system loses pressure (excepting auxiliary air systems, which are like a second smaller tree that only gets fed if the primary has enough air to spare). Getting capped at 15 PSI with the engine compressor running suggests a large leak, so my first suspects would be a blown air bag or tank (unless the compressor itself is shot).

First up, get a big spray bottle full of soapy water and spray it on every line, connection, tank, air bag, and brake chamber that you can see/reach, being mortally mindful not to go under the thing unless it's blocked up.

If that doesn't help, hooking up a pressurized air tank with the valve just barely cracked can help you track down leaks by sound with the engine off. The quieter the surroundings, the better.

Wouldn't hurt to do a dummy-check with an external air gauge to make sure it isn't an issue with the dash gauge.

If all else fails, you've got to start segmenting the system to isolate the leak, so you'll need some fittings and caps to assist in that depending on your hardware; start from the air compressor / main tank and work your way out, capping and testing to see which 'branches' hold pressure and which don't until you've found your culprit.

[bluegrassmidtenn]

Many thanks. I do have a service manual, and I'm a retired master plumber, so a lot of the things I see in there are familiar old friends. Appears to be several miles of various sizes of soft copper air lines running to every corner of the bus it seems. The fact that I'm getting closer to winter doesn't help my attitude.