4 hole bath mixer - shower head siphoning any fixes?

[kaner.dr]

I have a 4 hole bath mixer (hot, cold (quarter turn), bath filler, pull out shower head). All are set on a flat plinth next to bath. Frequently when turning on the tap (only) the shower head starts to leak water - perhaps due to a siphoning effect. Any suggestions on how to fix? Is it a problem with the design or a faulty part?

Thanks in advance

David

[stoneyboy]

kaner.dr,
You do not name the make but I would suspect the lever/valve which directs the water to the bath or shower is not fully turned or its faulty.
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