If its leaking water, then the thermostat isn't the issue. Also, the thermostat isn't opened by pressure, its opened by heat. Its basically a piece of wax that melts at like 190 f or whatever its set to. 200 f is probably in the normal operating temp range. My old 88 camaro's fan wont even kick on until 220 degrees. I'd start to worry about heat at like 250.