I made these to check an 8BA (even in the car). It can also be used with a vinyl cap on the lower water pump ports if you can't get the pump off. It's not the best, but it sure weeds out the badly cracked examples. Leaky water pump seals also render it ineffective when used with the vinyl cap. One thing though; if it holds pressure with this, most likely, it's good. Since the water passages on each side of a flathead are not connected, each side can be tested individually.
One problem with this particular example is that the aluminum water pump plates tend to flex and leak. I made special gaskets from an old inner tube which solved my problem. If I had to do it again, I'd use 1/2" thick aluminum or 1/4"steel.