86' Grand National PowerMaster Brake Unit, lite ON when applying brakes

Yeah, your corner brake shop won't be of much help with this system. Gotta be willing to maintain and troubleshoot it yourself (like you just did) or swap it out to a more conventional system and move on. Glad you got it figured out.
Yup exactly what he said. Your local break shop actually could do something to further damage the unit. Like not changing the fluid correctly and not gravity bleeding the motor reservoir. Thank God they were honest with you and walked away from the job. Not too many mechanics will play with powermaster units unfortunately.