If the thermostat is not Behr branded, it does not belong in the car.

The correct thermostat should have a bypass hole in it already. The likely cause of your loss of pressure is the expansion tank. I had to replace mine because it had hairline cracks that would lose pressure but not obviously leak coolant.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge