Yes I did test without fluid and it appeared to be holding air ok. Eventually used the supplied plastic cap reservoir adaptor as that had matching bayonet fitting from pressure vessel.

On my Roadster MMC had fitted a heat resistant cover over m/cylinder reservoir which inhibited the cap being screwed full down as it fitted over the threaded plastic. This protection was needed due to proximity with o/s exhaust manifold. Easily removed though as held on with cable tie.

Problem I found was that the bayonet connection would be air tight for a while then suddenly let go with subdued short hiss at which point fluid would leak out rapidly. Fortunately there is a pressure release but impossible to get to it quickly enough. It was if the rubber seal within bayonet was temporarily displaced as it would always seal again later for a while, albeit with reduced pressure.

Richard

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