I have a circuit tester for that, and have tested the sensitivity of the switch. Seems to take a reasonable pressure to activate, and is crisp in its on-off operation.
Rob, you mentioned the return port in the M/cylinder, and I was thinking along those lines: crud in the m/cylinder well, partially blocking a port. Looks like I'm at the stage of removing the m/cylinder, giving it a good clean and reassemble. What a pain!
Might pull a quick one and do a change over from the new spares box, then pull the old one apart to see what's what.
They are great when they are working fine, but ...
Thanks again, Gents, for your very helpful comments.
Mike