I'm lazy, and hate taking anything apart until every possible simple diagnosis has been tried.
Are you absolutely positively sure that there are NO air leaks?
At these levels, you need a helper, since the time it takes to get back there with a big leaker, it will be silent before you get to the back from the front.
Any oil in the engine room drain? No oil is a good sign.
If it periodically makes air pressure, you can rule out a drive problem with the compressor.
Put an air gauge on the drain in the engine room and see what pressure you are getting there. This will show whether there is a blockage upstream somewhere, as there will be strong pressure here if it is restricted. Same as the earlier postings with the corrupted air drier. You will get zero pressure here when the compressor is cut out and the drier purged, so you can't jump to the conclusion things are broken right away with a zero reading.
Do the same thing with pumping the brakes down and then watching the air gauge down the back to see what the compressor is trying to do. If the wet tank is not seeing the pressure to cut out, the signal line to the governor isn't getting to that pressure, and won't cut out the compressor, so it will keep running until the wet tank sees the pressure sufficient to cut out. The wet tank is blind to you, without adding a pressure gauge to the drain on it.
A new governor hasn't changed anything from the sounds of it, so perhaps the activating mechanism in the compressor is misbehaving, sticking, worn, partially broken,
. The governor is signalling, but the compressor is not always responding by cutting in when the governor acts on it to do so.
Maybe do a bench test on that new governor, since pulling it off and making sure you don't have a bad coincidence of a failed or improperly set new unit, is easier than pulling a good compressor off...remember, LAZY....
Fooling with the compressor comes after absolutely confirming that in fact it is the culprit, and there is no blockage, leak or lazy valving elsewhere causing the cussing.
If you determine the compressor is not reliably cutting in, remove and trade it in for a rebuild, and bolt in the fresh one and you are on the way this afternoon. Unless you have the background, or SHE doesn't mind being sidelined again because you rebuilt it wrong, don't bother with rebuild kits. Nobody who is making money with a truck or coach is rebuilding on the shop floor anymore, so why would we?
Bus conversions are notorious for the internals of the air system to get sticky, plug and jam up due to old stuff and LACK OF USE. Without the parts moving regularly, lubricants dry, metal bits corrode, rubber sticks like glue and otherwise stay where they are, and the air pressure or springs that make them move are not strong enough to overcome the conditions the valving now finds itself in.
Commercial duty cycles and the maintenance manuals did not for the most part ever consider the infrequent duty cycle or condition that we try to operate our coaches in.
A systematic replacement of all air system valving over some budgetary time frame would be an excellent strategy for a reliable bus conversion air system, since we have no way of knowing what abuse, contamination, ignored replacement schedules and lack of maintenance preceded our ownership.