Plumbing – How to remove this toilet supply line that seems to have no nut

Knowing where the water lands gets you in the area, but what you really need to know is where the water came from. Is the tank cracked, is the supply line failing, is there a bad connection on either end of the supply line, or is the valve itself going.

If the tank is cracked, you'll have to buy a new one, and it's a large project if you're not used to working on basic plumbing jobs or are skirmish around an open toilet.

A bad supply line can be easily replaced and it's even easier if it's just the washers.

A bad valve may just have some dirt or buildup in it that will be fixed by opening and closing it once or twice with water running through it (an empty toilet tank). Though keep in mind any dirt may then get in the toilet valve and could cause it to run slowly until you clean it out. If that were to happen, just post another question. It could also be the packing material in the valve, which I've never touched on this type of valve. If that's the case, you may be able to minimize it by opening the valve all the way. If the valve is too far gone, you'll likely need a blow torch and solder to replace it. This can be a DIY job, but if you get in over your head, the water for the whole house will need to be shutoff until you can get a plumber out.

There will indeed be some type of rubber, silicone or other type of flexible materal washer thing inside the nut. The cross section picture below attempts to show how the typical compression fitting of this type works.

As the nut tightens it compresses the "rubber" part against the end of the threaded plastic piece and around the outside of the incoming water line. Leaks can arise as from this "rubber" part getting hard and inflexible. The chorine added to water can accelerate this this problem. Over tightening the compression nut will also lead to problems. Sometimes a corrosive build up on the water tube can also lead to leak issues. Usually it is combination of all three elements and everything is OK till there is some flexure in the joint and then it starts to leak.

To fix it is suggested to replace the compression nut and the "rubber" part. If the inlet tubing is corroded then clean it up with steel wool. If the plastic threaded part is stripped from previous over tightening then you will also have to replace the fill valve assembly in the tank too. You should be able to find replacement parts at hardware or home stores. Replacement parts are usually fairly reasonable in cost.