Sealing off isolation valve

[Erico19]

Hi,
The isolation valve feeding my toilet cistern was leaking. After considerable effort, I was able to lift up the cistern to put an O-ring washer around the top of the inner circle referred to with an arrow in the attached image. Unfortunately, when I let the water through, there was a burst leak. Should I use a flat-ring washer instead? If so how can I get the right size?Should I put the ring at the top or push it down?
A friend suggested using also a jointing compound along with a PTFE tape; If a washer is enough, I won't buy these products

Thank you.

 

[ShaunCorbs]

1/2 fibre washer or a 15mm hep2 o flat washer

 

[Erico19]

Thank you, Shaun.
For the flat washer, would a rubber washer be sufficient or does it have to be a steel washer?
I take it, there is no need to use a jointing compound and/or PTFE tape, right?

 

[ShaunCorbs]

Rubber or fibre

No not steel

No doesn’t need any paste ptfe or anything

 

[Erico19]

Rubber or fibre

No not steel

No doesn’t need any paste ptfe or anything

Thank you for the confirmation, I'll let you know the outcome

 

[Erico19]

Hi,
I got the 1/2' Fibre washer today, place it as you can see in the attached images. Unfortunately, still the water leaked out. Any advice, please?

 

[adonis]

I find sometimes if you put 2 fibre washers on the fitting it can work.

 

[Basher]

The mating metal surfaces that compress the fibre washer need to be clean and smooth and damage free.
Surprised it didn't seal! Did you do up the nut tightly (but not so tight as to damage the plastic thread)?

 

[ShaunCorbs]

Prob a bit loose it’s hard to guage with plastic threads best would be to tighten up then turn the water on and tighten slowly until no water is leaking

 

[Erico19]

The mating metal surfaces that compress the fibre washer need to be clean and smooth and damage free.
Surprised it didn't seal! Did you do up the nut tightly (but not so tight as to damage the plastic thread)?

Thanks for your reply. I just put the washer without any cleaning...I upped the nut nicely and smoothly to align with the plastic threads.
I wonder if I have to put a bigger washer to fit inside the nut instead of putting a 1/2" washer at the top of the inner circle of the isolation valve? Should I use a jointing compound along with a PTFE tape?
Cheers