You say its loosing coolant,and it was pressure tested with no leaks found.I would be suspicious of head gasket problem.The coolant is going into the cylinder and being "burned" in the combustion process.
Look to see if your getting any white smoke out the exhaust,especially after its warmed up.
If you dont see anything,then take it in to a shop that has a hydro-carbon sniffer,have them run it across the radiator fill tank.If they pick up any hydr-carbons,then youve found your problem.Gasket is failing.
Btw,the sniffer I am referring to,is used for tailpipe emission checks.
And they dont need to stick it into the coolant.Hover it ,and it should do,the trick.