If you go this route, you may want to plumb in a check valve, between the PacBrake pump and the tee going to the solenoid and the tank, as the tank will leak down overnight, back through the PacBrake compressor. Will save you from having to use your pancake compressor every morning.
Haven't needed more than the 1. 5 gallon tank going down any of the long grades in WA, OR, ID, CA and NV. Only time I overheated the pump was climbing UP Priest grade in CA with temps in the 90's, didn't shut off the exhaust brake, and it cycled every corner going up. Exceeded its thermal cutoff point. Took more than an hour to cool back down and reset itself.
-- Bob