Might be tempted by Dowty washers Rob. Usually of relatively thick steel (which will help in your case) with proud ring of nitrile rubber bonded to the inner surface. They are ideally suited to high pressure hydraulics, with the seal improving with internal pressure.

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They may come under a different name in the US.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone