Pontiac Repair: 1998 Montana coolant fill procedure

Question
Hi Todd. My daughter has a 1998 Montana, 3.4L that started overheating a few days ago. I replaced the thermostat since that's such an easy job (NOT!) but that didn't help. I decided to replace the water pump so I drained the radiator and replaced the pump. The old one looked good when I removed it but I replaced it anyway. To refill the coolant, I cracked open the bleeder above the water pump and filled the radiator until coolant came out the bleeder. I closed the bleeder, replaced the radiator cap and ran the engine until the thermostat opened. I then opened the bleeder valve on the thermostat housing until mostly coolant came out. The engine overheated quite rapidly. Both radiator fans came on. I think I might have air in the system. Do you have the correct coolant refill procedure? I also rented a cooling system pressure tester from the parts store. I pumped it up to about 13 PSI and it dropped down to 12 in about 5 minutes. I'm also suspecting a lower intake or head gasket issue since I don't see any visible leaks. I don't see any coolant on the oil dipstick but I haven't drained the oil to do a closer inspection. Also,there's no heat coming out of the heater, front or rear. Sorry for the long message but I thought you should know everything I've tried. Thanks!

Answer
Hi Dan.
Bad news if you over heated this it more then likely blew a head gasket. more then likely the lower intake has been leaking for a while and ran low on coolant and then it over heated. causing a head gasket failure see that cycle all the time on those vans. you have the bleeding down right but it's more then likely getting compression from a blown gasket into the cooling system which is causing your rapid increase in pressure.