I have a slow leak of antifreeze that I can't find. A mechanic supposedly did a presssure check and said the pressure was OK. I put in a new thermostat and a hose to the overflow container. The heater core and radiator appear OK. No wetness in car from heater core nor is there fogging of windows. There are two lines running to the transmission which I know nothing about. Al hoses appear to be fine. HELP! The model is a 740.

 

So, by "leak" you mean unexplained loss of coolant, with no visible leaks? The coolant may be burned internally through a minor head gasket leak, or there may be external seepage which seems to happen to these Volvo engines most commonly around the water pump or the head gasket. Once the engine warms up the heat expansion seems to seal small leaks and evaporate any telltale signs. The two transmission lines to the radiator are for cooling the trans, and usually don't have much leaking issues. You can look at the oil cap for milky residue which would indicate water/oil mixture but this wouldn't show unless the leak is pretty substantial. Anyway, a competent mechanic should be able to find the leak...

 

After further study I think that it is the head gasket leaking. For now I am going tojust add antifreeze and/or water. IThe cooling system took 6 cups of antifreeze after driving the car 100 miles.Thanks for the help tedv.