[WKB]

These caps, perish rather quickly as Boxhead mentioned, that are used to block of water points should only be used for blocking off vacuum ports. I was going to buy these when I was building my AC and parts salesman at Repco told me about what they should be used for. He also mentioned that they could fail if used on the cooling system.
The problem is blocking off water ports that cannot be replaced with threaded plugs is hard as there is nothing I have come across that can be used other than these caps.

Warren