Well pressure drops when the engine is cooling. You need to test it when its cold. When my heater core went it spilt out the drain hole, not into the car itself. Really dont know how its a freeze plug unless its rusted to hell and it just shattered.

And buy a spec clutch. $180 new, stronger than a king cobra and supposedly lighter pedal from what I read, mines sitting on the shelf waiting install. Though at NMRA one vendor was selling King Cobras for $150