Class A F53 (Ford Chassis) brake question

[Plowtoy]

Trying to help my buddy figure out his Class A brake system. First off, I have been on him about replacing his rear brake pads (have been metal to metal for a while now). He only puts about 200-400 miles a year on this motorhome, but has devoloped another problem with the brakes he's not sure how to fix.
Apparently, back in the late 90's (his is a 99), the F53 chassis had the hydro-boost along with an electric booster for braking. He said that this past summer, he lost the electric part while the engine is running, but it works fine if the key is on, and the engine is off. I am an ASE certified mechanic and know pleanty about braking systems, but ive never heard of an electric booster.
Is this (the electric booster not working) a common problem? or do you think its directly related to the metal on metal brakes? (some kind of sensor?)
Any positive feed back would be appreciated, Dave

[SMS1115]

From my knowledge, the electric booster is only there to assist if the hydrobooster is not applying pressure to the brakes. ie- engine stall, low fluid in the hydro system, etc... I have worked on a few church school buses and had to do a lot of research on the hydroboost systems. RV systems maybe different, but I doubt it. So, if all is well, the electroboost part should not come on unless there is a problem with the hydroside.