This is purely a technical question for the experts. If a bus is running down a 6% grade at 60 mph on the jake and there is a mechanical failure (jake drops out or something breaks in the drive line) will the bus brakes be capable of stopping it? Maybe the truck drivers have more info on trucks as I see them running above 70 mph on the jake and I have wondered if they could stop in an emergency.
I actually had an experience similar to your question dropping off Donner Summit on EB I-80 into Reno in an MC-9 back in my charter days. Fully loaded, with 44 passengers and three full luggage bays. Cooling fan belt broke, took out the wire on the Jake buffer switch as it whipped thru the engine compartment. Never heard the noise of the belt breaking - one of the passengers in the rear of the coach came up and told me. Knew instantly tho, that the Jake had quit. Immediately braked sufficiently to drop from 5th to 4th (HT-754), then again braking sufficiently to drop from 4th to 3rd. At this point, held the coach with light brake applications (snub - 5 mph window) for a couple of miles down the hill until we got to the canyon overlook, where I stopped to see what happened. Had no problem slowing the coach down - because I immediately dealt with the problem.
Had I NOT been paying attention, it could have become a "Film at 11" story.
As for the fan belt, the company always kept a spare in a plastic bag in the battery compartment, on the HVAC filter side of the partition (those busnuts with MCIs will know what I mean). I simply changed the belt (no tools required w/ the air tensioner), and we continued on into Reno, arriving about 20 minutes late. While the group was enjoying the slots, I took the coach to LTR (now a name from the past, at the time a great carrier) where they repaired the jake wiring.
However, the main point is this: Yes, the brakes will easily bring you to a stop if the jakes fail. Just don't panic, bring your speed down so you can drop into the lower gears, take your time and stay in control.
FWIW & HTH. . .