So, I have been looking around trying to find the answer to this before posting a new post. I couldn't seem to find any definitive answer, so, having said that, here's my question. My 1982 300 TD wagon has stopped producing heat. First thing I tried was to have the monovalve replaced. Unfortunately that didn't solve the problem. I have tested the auxiliary water pump by applying 12 volts via jumpers from the battery and I can hear the pump running, so I'm pretty sure that's not the problem. My question is, can I just replace the pesky monovalve with a 90 degree brass elbow or ball valve? Seems like that would solve all the problems, but I don't want to cause any problems anywhere else in the system.