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.
Replace the valve seal and check the valve guide for wear.
Yes, you remove the rockers, add air pressure in the cylinder through the plug hole and use a valve spring compressor to remove valve spring. You can now change the valve seal.
If you are talking about a temperature and pressure valve on a water heater (pop off valve). It is controlled by spring pressure and a rubber seal. As pressure in the tank increases the tension exerted by the spring on the rubber seal is over come until the point at which excess pressure is released around the rubber seal. When the pressure inside the tank decreases the spring again pushes the rubber seal closed. Temperature of the water in the tank is directly related to the pressure.. higher temperature = more pressure.
Replace valve seal
The system has pressure on it but the switch can be screwed off. A schrader valve prevents the loss of refrigerant. Install the new one after you have put some Air conditioning oil on the O ring seal.
Its called a valve cover gasket kit and the job should cost about 300.00 Parts and labor.
The head, or top part of the motor, must be removed to replace valve seals. After the head is removed take it to a machine shop and they will mill the head to make sure it is flat and also grind down the valves and replace the valve seals.
Usually the shaft seal is installed from inside the compressor housing and is not serviceable. You would need to replace the compressor.
your pcv valve or hose is restricted causing excessive crankcase pressure
Oil Pump Check Valve. Replace oil pump
The purpose of a leak detection pump pressure switch is to immediately alter the system to pressure drop. These are most commonly associated with seal or valve failure.
What do you want to know? Take the heads off, remove the valves, and springs to get to the valve stem seals