Is my plumber right about needing a total repipe for my kitchen sink?

Your plumber is almost certainly right about the setup being a mess if it's a mix of old metal and newer ABS. The problem with just fixing that one leaky tailpiece is that you’re attaching new parts to a system that likely has **incorrect pitch** or improper venting. If the drain line going into the wall is higher than the outlet of your P-trap, or if it's perfectly level, water just sits in those pipes. That leads to constant clogs and eventually eats away at the seals til you’ve got another leak.

The dishwasher connection is another big red flag. If that wye fitting is installed on the wrong side of the trap or without a high loop, you end up with dirty sink water backing up into your dishwasher. It’s a common DIY mistake to just "make it fit" using whatever fittings are lying around, but drainage relies on gravity and specific pressure balances to work. Mixing metal and plastic also creates weak points becuase the slip-joint washers don't always seat properly against diferent materials.

Redoing the whole thing from the strainers down to the wall is acutally the safer move. It sounds like alot, but we're ussually talking about a kit of ABS or PVC pipes that costs under fifty bucks at a hardware store. The labor is where the cost is, but it's better than coming home to a flooded cabinet. You want a continuous run of the same material with a consistent **downward slope** toward the wall.

If you try to patch it piece-by-piece, you'll be back under that sink in six months dealing with the next leak. Get the configuration fixed properly so the dishwasher drains where it shoud and the P-trap actually does its job. It’ll save your subfloor in the long run.

Look, if you're allready in there, stop messing around with that thin 22-gauge brass and just grab an Oatey 1-1/2 inch Center Outlet Tee kit. Your plumber is likely seeing a tee that lacks a directional baffle. Without that internal baffle, the water from your left basin is just shooting straight across into the right tailpiece instead of heading down the trap. If that dishwasher is hooked into a standard wye without a proper 7/8 inch ID hose, you're asking for a backup. Ditch the frankenstein setup and go full Schedule 40 ABS. It’s a cheap fix in parts that prevents a $500 emergency call when that old metal finally corrodes trough.

I've seen this movie a hundred times. Back when we were still using lead and oakum, we didn't have to worry about these flimsy plastic slip-joint setups failing every six months. If that plumber is worried about the wall height, listen to him. People swap out an old shallow cast iron sink for a deep stainless one and suddenly the drain in the wall is too high for the new trap arm.

You try to patch that togather with a bunch of extensions and you'll have a permanent puddle in your pipes. If the water has to climb a hill to get out of the house, you're looking at a grease trap in the making. Better to open the wall and lower that sanitary tee now while everythign’s allready apart.

He’s prolly right about the wall connection being the weak link. To get a solid seal between that old metal pipe in the wall and your new ABS, you need a Fernco Proflex Coupling. It has a stainless steel shield that prevents the pipes from sagging or shifting, wich is usally why those mixed-material setups leak. I always use the Proflex 3000 series for these transitions. It’s superior to those cheap unshielded rubber boots that just bulge over time. That shielded coupling is the only way to ensure the pipe stays at the correct pitch where it hits the stack.

If he's talkin' about venting issues, ask him about puttin' in a Studor vent or an AAV under there. If the old setup didn't have a real vent in the wall, the water's gonna gurgle and drain like crap no matter what. It's way cheaper than rippin' out drywall to fix the main stack. I'd bet that's why he's callin' it a total redo. Don't just fix the leak—make sure the air's movin' right or you'll be dealin' with slow drains forever.

Quick update—I checked that tee and there’s definitely no baffle inside, so you guys were spot on about the water crossing over. But now I'm looking at the wall height like one person mentioned. My new sink is way deeper than the old one, and it looks like the drain pipe might actualy be higher than the sink outlet. Is that why he wants to rip open the wall?