If I understand your question , the valve is to purge air from the fuel line after service. Diesels are nearly impossible to start with air trapped in the lines.
Depress the Schrader valve on the fuel rail mounted on the engine. It looks like the valve stem on your tires.
You may be referring to the valve stem on each wheel.
Fit a new valve stem.
On top of the engine there is a fuel rail that's chrome looking it's a round looking pipe; Has a valve stem cap on a Schrader valve (like on a tire valve stem) take the cap off carefully use a flat tip screw driver against Schrader valve like you would do on a tire Schrader valve to release air from a tire if your doing these to release the fuel pressure(wear eye protection just incase it sprays towards you and rags around valve to absorb fuel wouldn't be a bad idea either); If the reason you need this is for changing fuel filter I don't bother to bled system from Schrader valve all I do is make sure fuel pump has not Ben energized with out engine running; With engine off a course take fuel cap off to release pressure from fuel tank do to they are a pressurized fuel system use a oil drain pan under fuel filter to catch some of fuel that will come out when disconnecting fuel filter. It also doesn't hurt to clean fuel filter and line going into filter first to avoid dirt going into new filter; on a filter I changed once had the date and mileage written on the filter.
Attached to the valve stem inside the tire/wheel.Attached to the valve stem inside the tire/wheel.
In the tire. It is the valve stem.
The valve guide is a precise hole in the head that the valve stem slides in. The valve is retained by the retainer and two valve locks.
The spline that lifts and lowers the part to shut the valve be globe or gate valve
If the refrigerant has escaped, get a a/c valve stem kit from a autoparts store. They cost a few dollars and come with a stem tool and a few valve stem core that resemble tire valve stem cores. If you still have pressure, you will need a special tool that costs about 40 bucks.
Rising stem gate Valve- Stem threads are out side of valve body and in no contact with fluid medium. Stem rises while opening the valve (by hand wheel). Since the stem threads are located outside obviously it needs some more vertical clearance space while opening the valve.Non Rising Stem gate Valve - Stem threads are inside the the valve body and it is in contact with fluid medium. Hence, lot of chances for fouling the threads and these types of valves only can be used in clean liquids and gases. Less vertical clearance space required while opening the valve.
If there is oil coming from the exhaust manifold it generally indicates a bad valve stem seal or a worn out engine. (too much crankcase pressure)
I worked at a shop and when we would put on tires and new valve stem we would first cut the inside off or at least sever it and then you screw on the puller to the valve and use the rim as a fulcrum and pull out the old valve stem. Then you should lube up the new valve stem stick it through the hole and attach the remover and pull it through till it snaps in place.