I even found a kit cheaply on ebay, it does work and includes an adaptor for current VW Group cars.
I should put it use on my older daughter's partners 1998 Lotus Esprit GT3. his "Lotus approved" workshop have changed his car over from "blue" coolant to red/pink/purple coolant and I'm asking him the question "is this okay for this car", he is finding that difficult to find out or prove, okay Comma still quote their "blue" version, but could be because that coolant was current when that car was built - time will tell - I've only asked the question and need him to find the answer.
Another, but OT issue, that Lotus Esprit GT3 has a plastic coolant header tank and another plastic overflow tank, which seemed strange to me, but it seems that the header tank should remain 100% full at all times, and it should have a double seal "recovery" version of pressure cp, ie a pressure cap that opens at say 16psi and forces the expanded hot coolant out and into the overflow tank which just has a sealed cover on it, and also an overflow pipe, so the pressure in the overflow tank is always at ambient pressure. My next question for him is "is that new cap you bought for this car got a
lower AND upper sealing washer AND a vacuum recovery valve on the base of the pressure seating part. Until now I never ever considered that there were two different types of pressure caps, and so far I've not managed to find an online listing of types or model numbers, Serck for instance don't cover pressure caps nowadays!
Edit:- since I bought that cheap coolant pressure tester kit, I've used it to sort of gauge when I've removed as much air as possible from a cooling system, but marking the reservoir with tape and a black marker with the pressure cap off and the cooling system cold, then apply say 15psi or whatever the system is rated for, and make a mark at the new
lower level, and then recording it! If/when I replace coolant, if there is still air in a system, the level drop when pressurised will be more while air is still in it.