I am checking to ensure there are no leaks in the cooling system after making a repair, so I pressurized the system with air to 15psi. It hold well, but after about 1-2 hours the pressure has dropped to about 7 psi.

Is this normal? I may have a small leak in the pressurization system, but I was quite careful in how I assembled it. Any suggestions wouild be appreciated.

That does indicate that the pressure went somewhere. Can you rule out a leak at the tester or where the tester hooked to the radiator?

I "snooped" the connections and couldn't find any leaks. Just replaced the coolant bypass pipe a couple of weeks ago and was doing a follow-up. I haven't seen any indictations of coolant loss. The engine and transmission oil are good and even after several hours with pressure on it, I can't find a leak anywhere.

It sounds normal to me that after the engine cools the coolant pressure will drop. :?

Your right. The pressure will drop when the engine cools, but this test is with a cold engine. You pessurize the system with air and then look for leaks. Any other time I have done this, the leak was quite apparent and it wouldn't hold pressure at all. This time it is certainly different.

My bet is a leaky LIM gasket.

That is what I am afraid of, but the leak is too small to get a conclusive diagnosis. I suppose I should just strip it down and replace the gaskets anyway. :(

If there are not any leaks in your cooling system, or the pressure rig you are using to press up the cooling system, then that 15 psi should hold indefinitely with no drop in pressure. period