I'm an older guy who specialized in propane conversions. After I retired I continued to work part time looking after a small fleet of propane powered dump trucks. Around five years ago the owner purchased 4 2007 GMC C8500 trucks with the Isuzu 7.8L 6HK1 engines. It took a while but I've got a pretty good handle on servicing and repairing these units. They work pretty well however a number of issues have plagued us. I'm looking for some help in two areas. These engines are TC and also have air brakes. The compressor is directly driven off the flywheel.
Problem 1: One of the trucks go through about a gallon of antifreeze every 3 to 4 days. At this time the system has been pressure tested and holds pressure without any loss. One time we waited 3 days to see a change and no drop at all. The EGR coolers were suspected originally and confirmed to be leaking so we removed them. Replacements were rare and hard to locate so we took them out. The coolant loss persisted. There is no evidence of an external leak. The unit was tested for a head gasket leak. No head gasket leak.
At this point I am considering two options..
1. Can coolant leak into the exhaust through the TC?
2. Could the air compressor have a leak and be pumping antifreeze into the air brake system?

The second problem is the SCV (suction control valve). We replaced one of these several years ago to cure a stalling and low power problem several years ago. A GM service bulletin addresses this issue. Everything worked fine. In the last year we have had the other trucks experience the same issue. When I installed the new SCV the engines would not start. Disconnecting the electrical connector the engine would start and run for up to a couple of minutes and then stall. Starting the engine and reconnecting the electrical connector would result in the engine quitting immediately. Very puzzling! I used injector cleaner and a few other things to clean the old SVC and they work and run fine for 6 months or so. We have tried SCV's from several sources with the same issue. I know at some point I will no longer be able to clean the old ones so I need to know where we are going wrong. I thought maybe a reprogram might be necessary but it's not required according to the NHTSA bulletin.

Any suggestions here?