Start it up with the thermostat covers off ,should get a decent gush of water ,about the same out of each of them , only need to run untill you see the water , ideally within a few seconds . If its ok fit your new stats .
If I was having cooling issues ( or thought maybe I did ) I would run it in a barrel so as to remove any possibility that a pressurised feed ( garden hose and muffs ) is masking any pumping / flow problems
