Shut-off valves leaking

All the shut-off valves at sinks and toilets in my house are as shown in the picture below. The valve opens and closes gradually as you turn the handle. Many or most of them leak when I have to shut them off, or open them back up. I think these were the cheapest valves on the planet when my house was built (pretty much my entire house was made with the cheapest materials possible). I am replacing a toilet at the valve shown. I would love some help with 1) Is there a way to stop the leaks in these valves?, and/or 2) a good replacement valve. As you can see, I have PEX tubing in my house. I was looking at a Shark Tooth replacement valve, which is ON or OFF, nothing in between. I like the concept, if it actually works. But I am concerned that I won't have enough of a PEX stub for the Shark Tooth valve after I cut-off the existing valve. Anyone have any thoughts - I am open to anything.

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You have enough room to cut of the fitting and crimp on a new one. You can even cut the crimp off without damage to the tubing if your careful. Try www.Pexsupply.com and you will find 1/4 turn shut-offs with a PEX connection end that you can crimp back to your PEX stub out.

Follow up

Necessity forced me to fix one of these leaking valves. I shut-off of the main water supply to the house then completely removed the valve stem (piston). There were two rubber washers and one metal washer on the stem. The metal washer had hardened deposits on it, which I dissolved away with a humidifier cleaning solution. I cleaned up the rubber seals, then reassembled the valve, which is now working fine - no leaks. Don't ask me why I didn't do this before. I still plan to replace these valves as I upgrade toilets and sinks, it's only a matter of time before they start leaking again.

You can also use a standard compression valve on the pex line. The only thing is you must either buy one packaged for pex, or buy the reinforcement insert ferrules separately. SharkBite also makes valves you just press onto the end of the pex. See page 6. http://www.sharkbite.com/_images/pdfs/SB_Brochure.pdf
If you want to use a common compression stop, you need the pex insert fitting as seen in the following brochure. http://www.brasscraft.com/PDF/400.50_G2CR09.19.39_QTurn_Angle_Stop_Compr_x_Compr.pdf You can buy them separately, or packaged with certain valves.

Those valves are often "generic" so the operating parts from a new one may fit the old valve, even if the manufacturer's name is different. You can get a new valve, unscrew the nut and remove the "turning parts" from your valve and try screwing the parts for the new one into it. Tightening the nut under the handle slightly will usually stop the leak.