98 s10 2.2 coolant system

After replaceing water pump and therm, coolant seems to flowing back into the reservoir tank? Whats wrong?

How's your rad cap. The stock Chevy are junk, and should be changed to a scant
There is also a small chance that they got you the wrong water pump.

A waterpump designed for a v-belt is designed to run 1 way, where a waterpump designed for a serpentine system is designed to run the other direction.

After calling a radiator specialist, he said a blown head gasket could do the same, and he could do a chemical test for 20.00 to see if there was any gas,oil or coolant mix. To my eye there is no coolant or water in my oil. But will soon have a pressure test done on a lift to find the problem.

Sorry I never thought about that, and being a 2.2L there is a good posibility of a fried head gasket.
They don't always mix oil with water, it really depends upon where they blow out.

If it didn't give you any probelms before the recent work there's a good chance that it isn't the head gasket.

$20.00 to check that is kind of outragous. It's about a 1 minute test using a rad pressure tester.
Another way is if your reservoir puffs out like a ballon the moment that you start the truck.
If it does your head gasket is toast

actually OBP , i think the test that his guy is refering to , is a carbon monoxide test of the coolant , theres a special fluid you put in a special pump , hold it over a hot rad\resivoir and pump it , and it sucks the fumes from the coolant into the pump , and the fluid will change color if it detects carbon monoxide , but will stay the same color if it doesnt , but if it does , you got a combustion leak on one of the cylinders somewhere , but still , not really worth 20$ , you would proaly pick one up at your local parts dealer , ask them for a combustion leak checker