These modular valve / supply lines are truly awful for the homeowner and are an almost guaranteed leak / replacement problem. They are used because they are cheap and VERY fast for the original installation plumber.
Typically they will fail in one of two ways:
- develop a leak at either crimped supply line connection (where the line is crimped to the valve or where it is crimped to the nut)
- the valve will weaken over time and will not stay in the open position.
In either case, repair is impossible and replacement is your only solution.
They will twist off by turning and pulling the valve counter clockwise. I always just cut the supply line off to get it out of the way. This video shows the procedure.
https://www.youtube.com/watch?v=sNTfM_vku5k
You can also cut them off, just behind the valve body. HOWEVER, make sure you have enough pipe stubbed through the wall to attache / glue a replacement valve.
You can then get your standard 1/4 turn shut off valve and supply line (for toilet) at Ace, Lowes, HD and replace this modular POS. However, make sure that the new valve is made to glue to CPVC. This is the tan looking plastic water pipe that is used in TV homes and not your garden variety PVC. CPVC also requires glue formulated for CPVC ... AND be sure you let it cure for a minimum of 30 mins before you turn the water back on to the house. CPVC glue DOES NOT bond instantaneously like PVC glue does.
I will PM my contact info if you want to discuss further.