fill my sink and let it drain - the water comes up in the shower - its lower and it preferentially going there instead of the waste stack - - but drains away slowly while gurgling through the sinks u bend.
I've had the inspection cap off the Stack T piece and looked into the waste pipe with no sign of blockages
i think i have 2 issues here - one the old geezer hasn't fitted a trap to the shower - hes just got the drain straight into a 90degree bend - so will sort that.
second - i need a air admittance valve somewhere -terminating higher than the highest u bend(sink) . - but does it go between the shower and sink wastes or after the sink waste (the system is - shower - sink Tee'd into the shower waste - vented soil stack does it matter *
where does one get an air admittance valve ? would an anti vac trap be what i want ? or would an anti vac u bend trap on the sink alone do the job ?
*i accept that i may well flood my house if i get this wrong but at least my neighbours wont be harmed in the doing so