how can you pressure test this system i have a slow leak that i cant seem to locate

Does the leak only occur in cold weather?

No I T Has Leaked All Season Very Slow

I assume you are having a water leak. If so this may be from the winter/ water dump system. If you have a little air left in there it may be enough to crack these valves open when the engine heats up. Pull the air relief valve when the engine is hot, and see if this takes care of it.

Mine leaks antifreeze in the winter when it is really cold. I believe there was a TSB from Merc on this where the hose clamps that hold the heat exchanger in place were installed too tight from the factory and need to be loosened and repositioned...or something along those lines.

Get a hold of a Stant pressure tester that most automotive shops use to pressure check cooling systems on cars. There is an adaptor that Stant makes that is designed to screw onto screw type cap systems that threads perfectly onto the coolant overflow bottle on a 496. Pump up the tester to about 13-14 lbs and let it sit for a bit. If the pressure starts dropping it indicates a leak in the system and you should see the orange coolant tell-tale leak where your problem is as long as the leak is external and not internal in the engine. This is how we check 496 coolant system integrity.
Hope this helps.

Regards,
Ray @ Raylar

thanx raylar leak is outside motor i see a small drip now and then it is just really hard to determine where it originates i think it may be an o ring thanx for your input