Do others find the Mamiya double cable release to be unpredictable in operation?

Works fine on some lenses, fires mirror and shutter simultaneously on others, won't fire either despite degree of pressure sometimes.

A further question: I've just received a KL lens and the "Mirror Up" control on the lens doesn't appear to require turning of the cable release socket as on C lenses, but moves out as the cable release is screwed in to reveal a red ring. As this puts the camera (RB 67 Pro S) into mirror up mode I'm working it correctly?

It's also fussy about the double release.

I'm happy to use a single release, but it's nice when things WORK.

Regards - Ross