Re: low coolant problem
My leak was where the hose went on the surge tank.
Yes, replace the pressure cap and see if that helps.
NO! NO STOP LEAK! Unless you just WANT to replace the radiator also! And possibly the motor when it plugs up the smaller coolant passages! Doesn't happen all the time, but it's better to fix the leak than to compound the problem.
Let's see - another possibility. Do you smell antifreeze or does the window fog up when you start the car? May have a very slow leak in the heater core.
That should have shown up with the pressure test, though.
Past that - it could still be any of a number of other problems. I'd bet the pressure cap is not holding pressure, though, so try that first.
335K miles on original motor and tranny (only motor work 2 x LIM replacement), Monroe SensaTrac struts front and back (new at approx. 300K miles), and Alero discs in back. LS fogs added with custom wiring. HID high beams, HIR low and halogen fogs. Raybestos Pro Grade front rotors and Bendix D747 ceramic pads on front.
Addendum: Rest in peace. Surge tank blew, motor overheated, I think she's lived a long and useful life.