Turns out it was a fairly easy fix. Apparently it's pretty common for the vacuum pumps on these Jake Brakes to go out, and when they do it is normally the diaphragm that fails. On ebay it will cost you about $370 for a new pump, but only $160 for a diaphragm replacement kit, which is what I purchased.
Simply loosen the serpentine belt off the pump, remove the 3 nuts holding it, and pull off the pump.
Then take off the 4 bolts holding the diaphragm on, and check inside the lower unit. There should be oil in the reservoir which you can change (standard engine oil will suffice, and it should be filled to about 3/4 of the way). Make sure you spin the wheel around to see that the cam operates correctly. Basically unless it looks burnt inside it should be fine.
Then replace the diaphragm with the new one and put it back on.
Hope someone else can find this useful