Mira 723 Puzzle.

[Steveb01]

Hi, My Mira 723 shower has an odd problem.
If turned off fully it passes a trickle of cold water. If I open the flow control half a turn or so the trickle stops.
All other operation is fine. I’ve had the shower 30 years and every few years it’s started dripping I’ve replaced the ‘o’ rings and cured the problem; a ten minute job.
But this time after replacing the ‘o’ rings I have this trickle when turned fully off.
I’ve taken it apart again expecting to find a split ‘o’ ring, but all is fine.
I’m thinking that perhaps the arbour that holds the ‘o’ rings is being screwed in too far and passing a port allowing a back flow.
I think the shower is well made and easy to fix so I’d like to continue with it.
Does anyone have any ideas?
Thank you.

 

[Steveb01]

Just in case anyone is wondering about this problem, here is the cause.
When using the repair kit of ‘o’ rings I found the third and largest ‘o’ ring to fit the arbour to be 0.5mm oversize on the diameter.
This allowed the ring to seal as the flow control was closing, interrupting the flow. But when fully closed the excess diameter squeezed it out of its seat allowing the water to trickle through.
A correct size ‘o’ ring was substituted and cured the problem.