I've told this story a couple of times on this site, here goes again- My first C body came with a recently recored radiator. All was good for the first few thousand miles, then the heater core started leaking. It was an A/C car, so had the skinny dual core, which at the time wasn't available new. So I took the old one out and brought it in for a recore at a local shop. Got it back, installed it, it leaked right away. Took it out again, brought it back. They called me and said "we tested it, and it doesn't leak". So I went in, and asked to see it tested. They had a huge tank full of water, and had air pressurising the core. I said "well it's not hot", they said "that doesn't matter". I said "well obviously it does". So the guy who actually did the job, who wasn't too happy with me questioning his work, yanked it out of the tank, hit it with a tiger torch, and dropped it back in. Bubbles started rising, apologies were doled out, and they fixed it again, and I installed it again, after thoroughly testing it myself with 180° hot water.
So I wouldn't bother making this tool which will be used how many time's? And may not even provide accurate results. If it leaks air while being tested, surely it would have leaked coolant before the test.
From my experience, if you have radiator corrosion, and the radiator needs recoring, or replacement, the heater core is not far behind, and with the new radiator installed, the heater core is now the weak link. At the 50 year/60K-100k mark, both of these items should be replaced, in my opinion.