Re: Washmachine drain leak
The waste line under the floor is probably, mostly stopped by heavy material like sand. Each load that has just a little heavy material puts it in the pipe. Over time if your sewer lines are not pitched just so, the heavy material will fall out.
If you do one load it will get through after a while, several probably overloads it and the water starts to back up. It is also getting higher in elevation when it does this and putting pressure on the old joints under the floor.
At the hardware store they have a balloon fitting for your hose. It makes a seal in the drain and blows water under pressure through the sewer pipe. This may move the sand along and ease the problem. If it dosen't work and the pipe still leaks, or leaks worse, you're not out much as you'd have to fix the pipe anyway.
Worth a try.
Waste not, want not.
Use it up. Wear it out. Make it due, or due without.