The little 15mm screw control isolators always leak out the screw when used. Recently I've come across two 28mm lever valves that leak out the spindle. And bottle vents always seem to leak. Does anyone make such things that are actually water tight?
Even the pegler slotted valves leak, had quite a few leak out the middle. Generally pegler is decent quality tho
We only use proper pegler ‘ballofix’ valves at work now. Expensive to buy but save a fortune in the long run: the cheaper pegler slotted isolating valves from Screwfix aren’t much good. Lever valves, pegler are ok but ideally Hattersley 100 valves, again more expensive but best quality. Flamco AAVs I find are decent, now and again one will leak but generally they are pretty good. Stay away from merchants own brand ones, or cheaper alternatives. basically u get what u pay for, and buying better quality now will make life so much easier in the future. Nothing worse than going to turn a valve off on a Friday afternoon and it leaks from the centre and u can’t find the main stopcock!
I will often close off AAV's, and if air collecting becomes a problem, using them like a manual bleed valve isnot too much of a bother. Honeywell AAV's can be easily replaced without drain down, and the non return valves with many cheaper units often have a spring loaded valve to assist quick replacement.