You wouldn't necessarily feel a surge of coolant with your engine idleing, with that test you did. These WPs don't flow much at idle. Are you losing any coolant from the resevoir bottle? Have you pulled your oil dipstick to see if it's overfull? If you don't find a visible leak when you pressure test it, then suspect your heater core or internal into the engine oil.I have a 2004 silverado I think something is leaking a little coolant but can't find the leak, when the truck is running you can squeeze the radiator hose and when you let go I'm not feeling any surge of coolant any help would be appreciated.