Of course, I fix that and the supply shutoff valve starts leaking, so I got a bonus leak #4 to fix.
In the submarine force we used to refer to this as "chasing leaks". Eventually you work your way upstream to the main valve that's so big you have to wait until you're inport (and probably shut down) to fix it.
The phenomenon is so widely recognized that the submarine force implemented a "small valve maintenance" program. The divisions who own the valves are supposed to exercise them and groom them at least annually to make sure that they fail a little at a time instead of catastrophically. Of course when this program is started it's sheer hell for the first six months until all the little valves get tweaked.
I've implemented similar programs at home, and sure enough the first six months were hell, but the failures eventually get worked out. The worst ones are the old isolation valves for the toilets that spew tiny pieces of rubber seating material down into the toilet tank refill valves. Now when I fix a leaky toilet flushing valve I shut off water to the entire house and check the toilet's isolation valve as well, which means I'm "maintaining" a valve that I hardly ever use. Just like the good ol' days on sea duty.
I'd love to do all these DIY plumbing fixes myself, but we live in a condo that has no shut-off valve to each unit, so whenever we need to do any work, we must do a full water shut-down of the building, of course by paying a hefty fee to the condo plumbing contractor. So far, I have rushed to install individual shut-off valves on all sinks and toilets whenever the condo corporation did a planned shut-down, but I still can't do anything myself anywhere below the shut-off valve, for example to replace a leaky shut-off valve. This pisses me off. The plumbing contractor apparently has no clue where the main water pipe to each unit is located, so if I wanted to install one during a planned water shut-down, I'd have to start cutting holes through all the possible walls...This pisses me off even more.
The municipal water company or the condo association may be able to trace the pipe underground from the street connection into your building. (They're supposed to provide this "free" service before you start digging in a yard.) If you're lucky then you'll be able to trace the pipe into the building and only have to rip into one or two walls.
A friend's condo had a shutoff valve in their garage, which of course hadn't been used for years. When I worked on their leaky shower (with another leaky supply valve in the bathroom) I shut the valve in the garage and finished the shower work with no problems. However when I went back to the garage to open the shutoff valve (a gate valve), the stem snapped off from the gate as I opened the valve.
That's when I learned that the condo had no individual street isolation valves. Not only that, the shutoff valve to our group of four buildings also leaked. By the time the condo association shut enough valves to reduce the water pressure in the friend's garage, most of the condo's 130 buildings were without water. During the second hour of this comedy the plumber finally stopped waiting on the association and just unscrewed the garage valve's unions, which started a "controlled leak" of about five gallons per minute. Then he screwed the new valve onto the street side (onto the stream of water). He shut it to make sure it was holding pressure, and screwed in the unions on the house side. Four hours of prep for about five minutes of work.
As far as I know the rest of the isolation valves in the condo neighborhood are still leaking past their seats, but nobody has taken the time (and spent the money) to shut the water off for long enough to fix everything.
