I seem to have a leak in the resevoir. I have replaced the thermostat and filled the system with coolant. I also filled the resevoir with coolant. Upon inspection the other day, the resvoir is almost empty but the engine temp never spiked and the system has been pressure tested with no leaks. Has anyone ever replaced the resevoir? How hard is it? Any step by step pointers?

 

I am guessing its not cracked you filled it and as the motor runs it heats up pushes out coolant. And air then when the motor cools it draws the coolant back in.
It is a closed system.
If you see no leaks it if it does not feel wet underneath. Or wet spots in driveway or parking lot .
I think you are fine just add some more coolant

 

sometimes when you open the cooling system you get air trapped in it,after you run the car and let it cool down the air has been expelled and the system sucks the antifreeze out of the overflow bottle. Try filling it one more time and see if it maintains the proper level.

 

Unless you see evidence of a coolant leak, I would doubt the reservoir is the problem. If you do see a leak in that area, check the tubing first. The tubing and it's clamps are more prone to leaking than the container itself.

 

I'm leaning towards the "Air Pocket" theory posed by David, these Ecotec motors are a bear to get the air pockets out of after a coolant change. Its one the few times I really recommend having the dealer do something, they have a vacuum attachment to pull out the air pockets in the block and head.

Not only can entrained air in the system give you a case of the "mysteriously emptying coolant bottle", but you can also suffer overheating issues if you're unlucky.

 

Let the car sit idling for 15 minutes, then look at the pavement under the tail pipe. If you have a few drops of water you are OK, if you have a puddle start thinking head gasket. Pressure testing is not fool proof.

The reservoir on a post 2006 is held by the one bolt that you can see and a "blade & slot" kind of thing at the bottom.