Buyers of used motorcycle or ATV gas valves are very often disappointed to find that they leak when installed. 

Why?

When bikes or quads are dismantled for storage the fluids must be drained from the tanks, carbs, etc.  Not only for safety, but because old fuel breaks down and can gum up the moving parts.  Mechanics will tell you that it turns to shellac.  Probably not quite accurate, but anyone who has stored their snowblower for a year or two and forgot to put fuel stabilizer in first knows what I mean.

So the bike has been taken apart and the petcock sits on a shelf for a few years until it is sold.  The new owner takes off their old, leaking petcock and installs the "new" one and it immediately starts to drip.   Now he's mad.  The shop must have sold him a defective item.  Right?   Probably not.

When fuel valves are removed from the tank or the tank they are on is drained, the rubber o-rings that are supposed to prevent leaks are exposed to air.  This causes them to dry out and shrink.  Also, if the valve housing is made of aluminum is will oxidize in the presence of air (oxygen) and can cause a "crust" to form.  If this crust is thick enough and in contact with the rubber o-ring it can ruin the seal.

What to do now.

Before you go back to the salvage yard, screaming that they sell crappy parts, you can do a little homework.  The o-ring probably needs to be rehydrated.  First, take the entire petcock a soak it in gas for 24 hours.  Just put it in a coffee can, put the lid on and leave it alone.  When the 24 hours is up, try installing it again.  More than half the time, this is all you need to do & it will no longer leak.

If you still have a dripper, you need to take the screws out and take it apart.  Remove the o-ring (rubber seal & diaphragm for vacuum operated valves) and put in some gas and use a stiff plastic bristle brush and some gas to clean the groove the o-ring sits in and all surfaces and replace the rubber parts.  Reassemble the valve and try it again.  In most cases, the leak will have stopped and you will be good to go.
 
There are lots of different styles of petcock, but I would say that 90% of them are rebuildable.  If you can't get it to stop dripping or you find that the rubber is dry rotted, you will need to get a rebuild kit. 

Remember:  most salvage yards do not take returns on used parts, ourselves included.  If you want a 100% guaranteed perfect part, please purchase a new item.  Junkyards can not predict the remaining useful life of any used item.  We expect our customers to understand that good salvage parts may last one season or 10 and to be familiar enough with their bikes to recognize if a specific part is correct for their application.