Bathroom tap causes kitchen tap to leak from tap tail

[greeni73]

I just fitted a brand new 4-in-1 kitchen tap. Everything is fine, working, no leaks.

Except...

When someone runs the hot water taps in the 1F bathroom, I get a leak down the tap tails on the kitchen tap.

I think it's the hot water tap tail, as the leak stops if I shut off the hot water isolator valve.

Why is this happening? And why does it leak only when the 1F bathroom hot water taps are opened up?

Any tips to resolve gratefully received.

Thanks

(Turning on the cold water in the bathroom does cause the kitchen tap cold supply pipes to jolt a little though thankfully nothing leaks - but clearly something odd happening with pressure or suction or something)

 

[Madrab]

Sounds like you have high water pressure and the seal on the tail into the hot outlet is passing when the tap in the bathroom is turned off and it's ending a shock back down the supply (1/4 turn tap?)
May need the tail taken out of the tap and the seal condition checked, then a little silicone grease applied before (re)fitting.