Pressure testing

It should hold pressure for a minimum of 15 minutes. If so, I'd suspect that you have air trapped in the cooling system. Some of these GM cooling systems can be a real bugger to get all the air out. Park the vehicle on an incline with the front of the vehicle pointing upwards. Like halfway in the driveway. Eith the engine cold fill the radiator to the tip top and wait. The coolant will slowly go down and you may see air bubbles coming up to the top of the filler neck of the radiator. Continue topping off the radiator like this until the coolant stops going down. It may take a little while so be patient. When you're done, put the radiator cap back on and fill the plastic recovery tank to the full cold line. HTH. -Jim