[Vwdiesel] Quantum questions?

[William J Toensing]

My Quantum TD is still loosing water & then overheating. Stop leak hasn't helped. The water loss seems to come from the coolant expansion tank which is now cracked. Suspecting a leaky head gasket, I bought a testing device at our local Napa Auto parts store. It works by pulling any exhaust gas that may have entered the cooling system into a device you put on the hole where the "radiator cap" screws onto the expansion tank. If any exhaust gas is present it is supposed to change the blue color of the test liquid you poor into the cap you place on top of the cap hole from a bright blue to yellow green for diesels & yellow for gas engines. Mine stayed a bright blue suggesting the problem is something else The prior owner said he took it to a local garage & were unable to diagnose the problem, which he said happens on mountain grades & hot weather. Temperature this afternoon was about 90 F. I think this is why he gave up on the Quantum. I wonder if a sticky thermostat or partially plugged radiator could be the cause? Something else?  When pressure builds up in the cooling system, shouldn't the expansion tank coolant cap release the pressure? I just put on a new one. 
I also say what looked like a leak from the rear of the injection pump with the engine warm & ideling. Will check to see if everything is tight . Anything I should check for?