I've asked plenty of questions around here. My turn with an answer, and a neat trick. I can't take credit for it; I found it on the internet.
To make a long story short, I had a tiny leak in the DWV for my new bathroom. In a bad place, too. I really didn't want to cut it out. I sealed up all the openings but one (lav sink drain) and sealed my shop vac in the pipe. My wife sat at the vacuum switch while I went under the house. When I touched the leak with the purple primer dauber, I could hear it suck primer in. Pffffffffffffft. Dab with the glue dauber. Pfft and FIXED!