How do you repair a water supply valve that is leaking. It is the valve that connects the water supply to my washing machine. The leak occurs when I shift the lever from off to on and vice versa.

You need to tighten the nut.

See this picture

http://img507.imageshack.us/img507/6229/onoffvalvezy6.jpg (http://imageshack.us)
http://img507.imageshack.us/img507/6229/onoffvalvezy6.dbc071602f.jpg (http://g.imageshack.us/g.php?h=507&i=onoffvalvezy6.jpg)

Knucklez has given you a picture of one type of valve but there are many types, usually washing machine valves are plastic knobs either 1/4 turn or several turns to close. Is there a nut under your knob or can you post a pic like Knucklez did.

Knucklez:
I don't agree 100% with your answer.

My answer:
1) Remove the handle by removing the first nut.
2) Tighten the packing nut underneath
3) Put the handle back on.

Hmmm... could be.

My plumber that came and installed a valve just like this on my mains and he was the one who gave me this advice. Says he does this on ALL of this type of valve even brand new out of the box because they have high probably to leak. I then watched him do exactly that.

Oh well.. can't believe everything you hear/see eh?

I agree, I talked to a mfr's rep about the leaking and he said the problem was unknown to him. They all seem to need tightening.

It's probably a good thing to sweat the valve with the packing nut backed out.

I would like any comments about sweating. Are the following statements true.

1) Loosen packing nut before sweating
2) Remove handle before sweating.
3) Remove drain cap, if equipped.
4) Sweat ball valves when closed and sweat seat type valves when open
5) Let valve cool before re-tightening packing nut.

The valve looks like a screw vs. a nut. I also think there is a rubber washer underneath the screw. The other problem is all of this is incased in a plastic housing so I can't fit a normal screwdriver above it to try and tighten the screw... do I have to detach the entire thing?

Below is a picture of the type of valve I have.

You can probably get away with using an offset screwdriver.

Craftsman Offset Ratchet Screwdriver Set - Model 4116 at Sears.com (http://www.sears.com/shc/s/p_10153_12605_00904116000P?vName=Tools&keyword=offset+screwdriver)

Under the screw you will likely find a nut that could be tightened with a box wrench or deep socket.

I have never seen a valve like this but it appears to have a brass nut on each end facing out. I suspect that you can access the seals by removing those two nuts but again never worked on this valve.