Coolant being pushed into reservoir, not sucking back in

[97Corsitruck]

1997 lumina, 3100, i had suspected a head gasket, but then found an external leak which i fixed, then put about 200 miles over the weekend in test runs, then today I took her out ran her for a good 2 hours, then she passed smogged, with very good results btw, but then on the way to work tonight, she heated up (there was air in the system so the gauge fluctuated constantly) and threw the low coolant light, when I stopped i opened the bleeder valve to relieve pressure, nothing but air, then popped the radiator cap, half empty, the reservoir was almost overflowing. when i popped the cap, it slowly drained back into the radiator, until it reached the bottom of the fill neck, then stopped. I ran it, bled some air out, went to drive, and it happened again. then at a stop light, steam briefly came out of my exhaust, i gave it some gas and it cleared up and the steam stopped, then i drove a good 4 miles back home to park the car and pick up the dually, and there was no steam the rest of the 4 miles, and the gauge bounced around but never reached as high as it had.

 

Is it possible i cracked the head gasket at the water jacket, sending pressure up,pushing the coolant to the reservoir?

[BXX]

Bleed the system out properly and put on a new radiator cap.

 

Remember, when adding coolant, open both bleeders and slowly add coolant until each one starts dribbling out coolant, close them, then keep adding coolant. Start engine with cap off. Open the bleeders and leave them open until coolant comes out, and make sure you keep adding coolant slowly to keep the level up. Once everything is good, cap the system with a new cap and let it warm up, make sure you have heat the entire time. Make sure temp stays still and you should be all good.

 

I do recommend a new thermostat though..

[3pt1lumina]

Sounds like what my old Malibu did when the thermostat went bad.

 

But I'd start with making sure that system is bled properly, as BXX said.