Derlin seal/valve head WANTED.

I’ve bought a couple of valve heads off gunpower UK and both have gone the same way in that over time a lip develops In the head where the stem pushes In to. End result – the stem doesn’t seat In the head correctly so you can’t shoot. I’m wondering If anyone makes any decent seals for the stock airforce valve system – something that won’t go tits up In a few shots 😆 I tired drilling the first head and took too much off and don’t really want something I’ve bodged together holding half a litre of compressed air @ 200 bar 😆

Anyone make them and got one to sell me or know where I can get one??
ATB,
F.