Thanks for the feedback guys. What I found curious is that 4 of my brake units work fine (They are still holding air today after filling yesterday and don't seem to leak with brake applications) but I just can't fix two of them.
Steve's hint that larger id brass tubing is the culprit is the most plausible as the leaky units show very little resistance when the piston is slowly pulled out. Both units hold air when freshly lubed but soon start leaking after a few cycles.

They will go back to BVM who, I am sure, will sort them.

Jason, I do like proportional braking, saves tyre wear BUT I do like the chance to stands on the brakes if need be ! The Smooth stop when working normally does a very good job of this.

David, never used this valve on brakes , but Wren use it as gas regulator, nice device as used on my SuperSport and XL 200.

Other brake units I use are PST Proportional, Jet Tronics, and Ultra Precision all work fine and have proven 100% reliable.

Thanks again.

David