if its loosing coolant but cannot find it it maybe the water pump. this is inline with the fan belts as viewed from the passy side, but it is hidden by the belts a leak is nearly impossible to spot even on a ramp with the bottom covers off.
have a read of this link to see how/where it commonly leaks. its best spotted with a dentist angle wead mirror and a torch.
http://www.igmaynard.co.uk/bongo/forum/ ... ak#p572266
a pressure test doesnt always find a leaking water pump as the pressure swells the seals, they usually leak when cooling and the pressure is down or relaxed.