We have a few units that leak cooling water past the butterfly valves. If we are going to have a cyl head off for a while, we drain the radiator and engine together to prevent flooding the engine with coolant. We haven’t changed any valves, as it is a manageable problem. Our best practice is to have two 2×4 boards cut-to-length that hold steady pressure on a closed valve handle. The handle locks are broken on some of ours, and the boards hold them tighter anyway.