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The ball-cock valve in my old toilet cistern is leaking, so I decided to replace the valve and the arm. The plumber's merchant guy suggested I might only need to replace the rubber diaphragm in the valve so that's what I've tried to do but it's still leaking. There are a couple of parts in the valve and I'm not sure I've got them the right way round. Please could someone advise on this.

1. One side of the diaphragm has a bit at the centre of it slightly proud to the rest of the surface. Is it this side of the diaphragm that faces the arm and the float?

2. Between the diaphragm and the mains feed inlet there's a plastic part that isn't exactly cone shaped but it tapers at one end. Am I right that it's this tapered end that the diaphragm will press against to close off the water supply? This part with the tapered end had a plastic washer with it that I managed to lose under the floorboards. Can I use a rubber one of the same size etc instead? And I'm sure this is a dumb question but am I right that this washer goes on the flat face of the part, not the pointy end?

Hi zacccc
Normally the washer is fitted with the convex side facing the arm and float. The assembly at the other side of the valve body is valve cone facing inwards (pointy end in first) then washer (rubber should be ok) then rear body section.
Regards S

Thanks very much for that concise explanation. As it happens, I've fixed the leak now and I think I did put the valve together in the way you recommend so your input has made me even more confident. Thank you.