wham48

I was in a front end collision last month which led to a new waterpump and radiator. Ever since then my coolant has been "disappearing" (also the temperature gauge in my car doesn't work). Initially I noticed the water boiling out of the reservoir, so my mechanic put in a new thermostat. However I'm still loosing half a tank of coolant every 50 or so miles. We have also looked for any visible leaks--their are none; and the pressure is fine. Any suggestions?