Leaking bottom fed toilet cistern ball valve entry

[brian_mk]

A few days ago I noticed a leak from around the nylon backnut underneath the toilet cistern where the ball valve feed enters.
The actual ball valve seemed ok and I had a spare unused rubber dome washer for the valve entry hole so I simply cleaned things and replaced the rubber washer. I made sure the hole was clean by scraping and using a scotchbrite to remove any limescale.
The job included draining the cold tank in the loft. There is a gate valve in the cold feed from the tank but I don't trust it.

That initially appeared to fix the leak but I left a plastic container underneath just to be sure.
After a couple of days I found a couple of drips in the container.
As before it appeared to be from around the nylon backnut rather than where the 15mm copper pipe joins the plastic.
I suspect the root cause may be that the copper feed pipe does not accurately align with the hole in the cistern.
I think the copper pipe that emerges from behind the bath and joins a soldered 90 degree fitting is perhaps 2mm too long.
When I look at the original rubber washer, you can see that the indentation mark is not perfectly central. (See photos).

I think of a few options...
1) tighten the nylon backnut a bit and risk damaging the plastic threads. I could do that without draining the cold tank.
2) Get a plumber in to unsolder the 90 degree fitting, cut 2mm (maybe a junior hacksaw) and resolder it. Hopefully the dome washer would then sit centrally in the hole.
3) Cut the end of the copper and replace it with a short length of flexible pipe, perhaps include a service valve at the same time to make things easier in the future.

Any advice?

 

[brian_mk]

What if I were to drain cistern, loosen the nylon backnut, dry the area around the valve entry hole with a hair dryer, apply marine grade silicone sealant between the dome washer and the hole before re-tightening the backnut. Would that work?

 

[ShaunCorbs]

Did you replace the washer between the copper connection and the ball / float valve ?

And any reason why you couldn’t get it central in the hole ?

 

[brian_mk]

Did you replace the washer between the copper connection and the ball / float valve ?

And any reason why you couldn’t get it central in the hole ?

There was no need to replace a washer between the copper connector and the plastic. I am not even sure if there is a washer there. That joint was not leaking before and is not leaking now. The leak is between the rubber dome washer and the cistern.

As already explained, the vertical feed pipe will not sit centrally in the hole because the copper pipe is not properly aligned. If you fit the plastic pipe to the cistern first, making sure it's central in the hole, then the copper fitting cannot be pushed in: There is not enough give in the copper pipe. The only way to connect it is to losen the nylon backnut which allows the plastic pipe to move allowing the the copper fitting to be connected. Unfortunatley that pushes the plastic pipe and dome washer to the left slightly such that they are no longer central in the hole.

I suspect that when the toilet is flushed, and the float reaches the end of it's travel, the additional weight pulls on the vertical pipe allowing a just a drip or two of water to find it's way behind the rubber dome washer. I don't see any evidence of a leak if the toilet is left without being flushed for a couple of days.

 

[ShaunCorbs]

The washer either rubber or fibre needs replacing after you’ve disturbed it unless it’s brand new

You should have enough flex in the vertical copper pipe to move it forward 2-3mm

How tight is your plastic Cistern nut as this being loose would be effected by the float moving