[MrE]

I can't help you with the Jake brakes, mine never worked on my 8V92 before it blew. I do know about compression braking on motorcycles, a four-stroke slows down better letting off the gas than a two-stroke with a compression release. It sounds like although you're getting juice to the solenoids, they're not working.
Congratulations on the front AC! That will make it nice, seems to be an issue with a lot of Newells.