That was another weird issue I had.
When I first removed the heater I also closed the two coolant valves going to/coming from the heaters. I placed the hose lines I removed from the heater in a 5 gallon bucket for the remaining coolant to pool into. I was only getting coolant coming out when I drove the bus. I expected it to be some remaining coolant from the lines. However... the coolant only seemed to come out as the bus warmed up and pressure built up in the lines. That would only happen if the lines were "open" to allow the flow of coolant. Yet... both valves were closed....
That boggled me.
I opened both valves yesterday and replaced the coolant that was removed when I took out the heater. I let the bus idle for a good 30 minutes as I waited for any change in the heater to warm up or what have ya... and nothing. I am going to play around with it today I see if I cannot find out my issue.
I first thought it did not matter what direction the lines went into and out of the heater... but I have a feeling there is a set "In and out" for it. I am also going to check the lines and see if there is not something going on with the flow somewhere.