header tanks cistern valves

A novice here: just returned from Hols to discover water jetting out of overflow. After pondering where and why - its the cold water header tank float valve. It didn't cut off - so I consulted DIY book and identified need for new washer. After some time I installed this - and indeed the flow is now stemmed. However - the thing doesn't really ever cut off. I've tried adjusting the float (if I manipulate the thing it does cut off, but I suspect my manula pressure is more than the water?). Does this mean I need to replace the valve itself (ie the brass bit)? I do notice that the whole float valve is not quite at 90 degrees (so may be the pressure of the float is not effective. The question is then - how on earth do I work out what type of replacement valve I need (size etc)? The washers were easy because they came in a pack of 3! Or shoudl I just get a plumber to come?

99 % of balvalves on watermains are 1/2" & brass. take a good look & go get one .but not the best time to be doing it if you get out of ya depth. If brass it will be a part 1 or a part 2 as long as its the same it will be fine. buy a roll of ptfe if the tapconnector washer is shot at. if u get the same type of valve its often possible to leave the tail in the tank & attach the new at the disco nut. making sure to use the new seal.

It is actually easier and quicker to fit a new valve than to re-washer the old one. You can adjust the level of the float - you want it set lower so that the water cuts off before it gets near the overflow.

Get plenty of light so you don't put it in cross-threaded or anything. As bab says you can leave the part that fits into the side of the tank in place and screw the new part onto it. Clean off any old PTFE or compound first.

Take the new valve apart on the kitchen table first to familiarise yourself will all the parts and method of assembly including any washers, and how to wind new PTFE on the threaded tail.

After swapping the valves you can take the old one apart, check the washer is correct, reassemble and put it ready to swap back next time you have wear in the one you've just fitted. I like to keep a new ball valve ready to fit in the loft next to the tank.

Thanks to Bab and JohnD. I think we'll survive until the world's back in order. The house is pretty old so I'll have a go at replacing the valve when rescue might come if it goes pear shaped. I'm not quite sure why the thing doesn't work though: the float is set to cut off below the outflow - might there be something in the area of the seat/nozzle - so the washer doesn't fit properly? Or how about if I don't have the mains tap open at full bore?

on modern valves the washer seats against a plastic orifice that commonly get hair cracks chewing up washers & letting water seap past.

Prof. what`s manula is it like micturition Sure it was washers in the pack of 3........and you want a plumber to come.......I see