I bought a couple of these spare and thought I would take the old ones out just to check the spares I bought were the correct fitting - they were - but on re-fitting the old ones and inflating to full pressure the air rushes out when the pump is removed, fiddling around with the valve stem sorts it but on re-pumping the same again.
These are the new ones below, they are slightly different with a small flexible nylon disc held with a screw as the seal, the older ones have a rubber disc with a spike that sits loose in the valve.
ZODIAC BOMBARD Boat Plunger replacement AIR valve body | eBay UK
Anyone had this before or know any trick, I have checked and double checked the valve and rubber is sitting right, the valve insert is fully screwed in, the stem is 'out' etc.