I was just worried about where I screwed in the sending unit possably leaking, any time I thread something into plastic I get conserned about a leak. The car is'nt drivable the way it sit's right now, so I can't take it out for a drive to get the S/C system temp's up to check for a leak. But after thinking about it I thought pressure is what I need and since the systems are tied together at the overflow resevior, the heat exchanger sould have the same pressure as the radiator. I was just wanting somebody to confirm my thoughts, and hoping someone could tell me when the S/C pump come's on because I couldn't hear it running.