Actually on a cold eng test pumping it up to 15 lbs with air is just as effective as pumping it up to 15 lbs using water/coolant & air keeps the messy coolant out of there till we find the leak. Adding coolant then firing it up would let you check for combustion gasses which'd pinpoint it to the head gasket/deck surfaces but it'd sure be nice to find/fix the leak before dealing with coolant. I spoke with a shop once about searching for a vacuum leak & they said something about they use a smoke producing machine to introduce smoke to visually spot the leak. It'd sure be nice if you could pump 15 lbs of smoke in there & see where it exited & yes I'd tighten up all coolant connections & pray that that's it & yes with it dropping pressure that quick unfortunitly there there is still a leak somewhere. EDIT Make sure the seal on the rad cap part of the pump kit is clean/wet so that it's not leaking there. A spray bottle of soapy water may help you spot an external air leak (Windex might work) on the hoses etc