I have installed a new kitchen sink, but the slip joints (brass) of the drain pipe leak, even though they have rubber gaskets and I have tightened them with a wrench. I did not use any sealing compound (pipe thread compound). Should I, or are the slip joints designed to function without such a sealing compound?
Thanks!

Yes. Compound should not be used on them. And be careful when using a wrench. Too tight and they will leak.

Take it all apart, clean everything well, be sure the rubber washers are in good shape.

You said new sink. Is the plumbing new, too? If not, new washers will be no more than a couple bucks.

Screw the nuts on as tight as you can by hand and see if it leaks. If it does, then use a wrench to turn it not more than a quarter turn or so.

Thanks, Rick. I have done it just the way you said (hand tighten, 1/4 turn by wrench), but I left the old washers, since they looked OK. Perhaps silicon tape will help...

You might have some luck with the tape (teflon). Even though they are designed to not use tape, sometimes I've resorted to it successfully.

"are the slip joints designed to function without such a sealing compound?"

Yes they are. Slip,( compression) joints make a water tight connection by compressing the rubber gasket. Since it's the gasket that seals the joint and not the threads no teflon is needed to insure a good joint. In all the slip joint installations I've put in I've never found it necessary to use pipe joint compound. If you have leaks are are using the old gaskets because,"they look good" then I would replace the old gaskets. Have a great day. Tom

Are Fernco rubber couplers (with steel bands) acceptable where the slip joint couplers leak?

No. The boots are designed to go on smooth pipe, and yours will be threaded on one side and have a lip on the other.

A package of a half a dozen rubber washers is only a couple bucks... just be sure to get the right size; Yours is probably 1 1/4" or 1 1/2"... take one in with you so you can be sure.

Sonething's wrong some place. My bet's that you have a 2" stub out with a 2" compression fitting on the end to pick up the tail of the trap. ( How'm I doing so far? ) So far so good! However a 2" compression fitting has a 2" gasket that you're attempting to connect to a 1 1/2" trap tail. So it's no wonder you have a leak. They make a 2 X 1 1/2" nylon compression gasket that fits a 2" compression fitting and converts to 1 1/2" to pick up the trap.
I apologize for not snapping sooner that you had the wrong gasket in the compression fitting, but hardly anybody has a compression leak problem and it took me a while to think it thriugh. Pick up and install the correct size gasket and I bet your troubles will be over. Let me know if that was it. Tom

Actually, the leak is NOT from the trap or the connections to it, but from the (angled) pieces of slip-joint pipes used to connect the sink's (threaded) drain to the straight brass pipe that is connected to the trap. These pipes (both straight and angled) are all 1.5" thin wall pipes. I have tried both the rubber gaskets and the plastic ones that are usually used with PVC slip joint connectors, with similar (disappointing) results.

Angled? Make sure the brass ends are aligned right and don't have any nicks or burrs in them. The tail stock that connects to the sink must be allowed to come straight down. Do you have more than the tail stock down from the sink, the ''U'' shaped trap, and the wall piece that makes a 90 from the trap back to the wall? Are the leaks in pipes connecting 2 sinks?

If the threaded tail piece from the drain does not line up with the trap raiser then you have just put your finger on the problem. Your problems not with a compression connection but with the way the trap's configured. If this had been explained from the git-go then labman, rick or myself would have picked up on it. The fittings in a drainage system must line up. They can't be cocked and expected to hold water. Go back and line the trap and raiser up with the drains tail piece and the problem will be solved. Tom

All I have is the threaded end of the sink drain connected to two angled pieces (the trap opening is not straight under the drain, but is a little offset, hence the beed for the angled pieces of slip joint pipes). There is only one sink.
There is no link when I run the faucet in the sink-- the joints leak ONLY when I run the washing machine, the drain of which joines the drain pipe from the sink.

All I have is the threaded end of the sink drain connected to two angled pieces (the trap opening is not straight under the drain, but is a little offset, hence the beed for the angled pieces of slip joint pipes). There is only one sink.
There is no link when I run the faucet in the sink-- the joints leak ONLY when I run the washing machine, the drain of which joines the drain pipe from the sink.

You don't have much choice, my friend. The reason you don't have a leak when you run the faucet is that the water is draining and not under pressure, while a dishwasher discharges into the branch tailpiece,(your angled piece) under pressure from a pump. As I said, you haven't much choice. You can duct tape the pipes till the cows come home but until you reconfigure the fittings so they line up and are not cocked or angled you're going to have leaks.
Regards, Tom

Thanks. The way I solved the problem (at least temporarily) is by using a Frenco sleeve as one segment, and tightening the other connections. No leaks in either direction now.