Your best bet would be to get a hold of a pressure tester and put it on the reservoir. Start the car up from cold and if the pressure shoots right up you have a cold compression leak. That could be a head gasket.

If you have a cracked head, the compression might not get into the cooling system till the engine warms up and everything starts expanding.

You can also look in the reservoir for any signs of foreign material, this can be a sign of exhaust gases.

Don't rule out the air pocket, and it probably wouldn't hurt to pull the water pump and check it out.
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