My bus had the exact problem. The rear heater was removed and the lines were plugged. The front heaters did not work. I reinstalled the rear heater and then all of the heaters worked. I then realized that all the heaters were on a single loop. By plugging the lines to the rear heater, this stopped flow to all of them. If you don't want to use the rear heater, replace it with a hose coupling.
79 International Wayne