Matco model MCR103U uses shop air to create vacuum (using a venturi valve) with the conical rubber stopper initially held down in place by hand, and the vacuum holds the stopper in place after the initial vacuum and refill, then it loosens up just a bit as the coolant refills (but usually not enough to allow air to get into the lines). Model MCR103A uses the old "standard" Stant / MotoRad (or a dozen other OEM versions, incl the truck brands) locking radiator cap and a variety of adapters for various models, which you may already have. (MotoRad bought out some of Stant's automotive pressure lines around 2020). Mac also has an identical version, just labeled differently.
