why is air bubbling back through coolant resevoir on 1991 GMC Suburban 2500

air bubbles back through coolant resevoir constantly pushing coolant into the reservoir & not allowing it back into cooling system thus causing "low coolant" light to come on.this happened after an overheating episode when i noticed my temp go up to about 210 briefly. i immediately replaced thermostat & radiator which was leaking a little. the bubbling reservoir started after these repairs.

Asked by for the 1991 GMC Suburban 2500
I'm suspicious you have a blown head gasket or two. A mechanic could check for combustion products in the coolant fumes to detect carbon moxide (CO). If there's CO ion the coolant, you have a head gasket problem. I'd check that first.
Here's a directory list for you: