Why is my sprinkler shutoff leaking after I turned the main water back on?

That square-looking plug is likely just a drain port, and since it only started leaking after you messed with the main supply, it's ussually just a seating issue. If that red handle is a ball valve, it looks like it might still be in the open position. Since it's for your sprinkler system and we're not quite in watering season for most people, the quickest fix is to just turn that handle 90 degrees to the **closed position**. If the line is plugged further down anyway, closing the valve shoud stop the pressure from hitting that plug and kill the drip instantly. If you actualy need that line open, you can probaly fix this yourself with a pair of wrenches. Those steel plugs are notorious for corroding or losing their seal when the pressure shifts. You'll want to shut the main water off again, unscrew that square plug, and wrap the threads with some fresh **teflon tape**. Better yet, spend three bucks at the hardware store and replace it with a brass plug. Brass won't seize up or corrode like the galvanized steel ones do over time. This isn't a sign of your whole system failing, so don't sweat a massive repair bill. It's just a common annoyance when old valves get jostled after sitting still for years. You definitly don't need an emergency plumber for a few drops an hour. As long as the water isn't spraying or pooling, a rag or a bucket will buy you all the time you need to grab a wrench.

But honestly, just try turning the handle first. If that valve is meant to be off for the winter, having it open is exactly why it's weeping. If closing it stops the leak, you're done. No need to overcomplicate it or risk snapping a pipe by cranking too hard on an old fitting if you don't have to.

Check the packing nut right under that red handle before you go ripping things apart. If it’s an Apollo 70 series or a similar Watts LFFBVS ball valve, the seal often shrinks over time. Use a crescent wrench to give the nut a tiny quarter-turn clockwise. That usally stops a 'phantom' drip instantly. If the leak is definately the plug threads, don't just use the cheap white tape. Grab some Blue Monster PTFE or a tube of Oatey Great White pipe dope. Those 1/8" NPT drain ports are picky, and a real sealant makes a huge difference. This is a five-minute DIY job, definitely not worth a $200 service call.

Thanks for the tips. I realy don't know much about this stuff, so I'm a bit nervous to start cranking on things with a wrench. If I try the teflon tape idea, do I need to worry about pieces of the tape getting stuck inside the pipe and clogging up the sprinklers? Also, is there any chance the pipe coud just snap if it's really old and I pull too hard? I'd hate to turn a tiny drip into a geyser because I'm a total newbie.

If you're worried about tape shreds getting into your valves, just grab a small tube of RectorSeal No. 5. It’s a yellow soft-set dope that seals way better than tape on those old galvanized threads and won't clog anything up. To keep from snapping the pipe, use the "two-wrench method." Hold the valve steady with one wrench and turn the plug with the other. If it feels stuck, spray some PB Blaster on it and let it sit for ten minutes. It’ll break the rust loose so you don't have to go full gorilla mode on it and risk a bigger break.

Listen to the guys telling you to use two wrenches. I’ve seen too many youngsters snap a riser right at the threads because they were pulling one-way on a stuck plug. Back in the day, we didn't have all these fancy tapes; we just used a bit of pipe dope and some horsehair if the threads were real chewed up. If that plug is truly fused, don't be a hero. Hit it with a torch for thirty seconds to expand the housing, then she'll spin right out. Just keep a wet rag handy so you don't cook the seats in that ball valve.

Quick update — tightening the packing nut worked for the handle drip, wich is a huge relief. However, that square plug is still leaking a tiny bit. I’m going to try the RectorSeal No. 5 next. Do I need to shut the main water off again to pull that plug out, or will the ball valve handle it?