Don't know about your bus specifically, but there could be bleeder valves at all sorts of high points in the system.
And as Smitty says, look for shut-off valves. They usually look like garden hose faucets. (but not always)
Finally, see if the temperature control valve moves when you move the lever (or knob) on the dash. On my bus, that cable appears to have been stuck open since WWI -- yours might be stuck shut.