DIY Doctor UK DIY Forums

A customer of mine has been through 2 Bristan basin mixers, same problem, keeps leaking after about 2 years.
Whilst I was there she also explained that when they run a bath on the 1st floor and then open the mixer tap on the 2nd floor there is a hiss and a thud as if the water has been drained from the pipes and is refilling. I think this is causing the fault to the mixer tap, but I can't understand why the pipes would be draining when the bath has been used.
They have an un-vented system.
Any ideas?

Do they have a poor water supply to the pressurised cylinder such that the fast filling of the bath is actually putting the pressurised cylinder under a slight vacuum - hence sucking followed followed by hammer on the mixer?