Plumbing – How to find the valves on a toilet with hidden pipes

It seems as if you have an obstruction that may be floating around in the pipe or valve. Sometimes it obstructs sometimes it flows clear. I would start with changing the water valve. If this is the old style "gate" valve there are several things that can clog the line. I would replace it with a 1/4 turn ball valve. That will give you much better flow than a gate valve. You will need to shut down the water probably at the main. The old valve is probably soldered, but you could cut it off and use a compression type valve instead. "Sharkbite" is one such supplier. Although I have never used their compression valve, I have used their end caps and found them to work well.

If you can have someone hold a bucket under the pipe once you cut off the valve and then a second person turns on the water main to fill the bucket to purge the line of any debris. With cast iron pipes it is common to get clogs from rust build up. The rust may not pass the small opening in the old gate valve when you first purged the line.. Although you did not mention what type of pipes you have, if it's copper than you won't normally find rust obstructions..

Are you sure it's actually leaking from the bottom seal and not just flowing down from the top seal connected to the cistern? To be sure run your dry hand around the pipe near the top seal.

In any case to replace any of those seals:

1. Turn off the inlet tap to the cistern and flush until no more water remains in the cistern. (Have some rags ready anyway as there's bound to be a small amount of water coming out residual to the cistern.)
2. Undo the nut at the cistern end by hand and lower this end of the pipe--now you can slide it back towards the cistern to remove it from the bowl.
3. Replace any broken seals and reinstall (reverse of removal instructions).
4. Open the water to cistern inlet and flush several times to check for leaks. Make sure both the cistern and bowl are well secured as any possible movement left may cause leak during usage.