It depends on where the leak is coming from. If it looks like it is coming from the side of the valve it might be best to replace the valve. Attempting to repack a pressure valve can lead to a bigger problem if not done right. You might want to call a pro.
Replace the valve seal and check the valve guide for wear.
The fix is replacing the leaking part.
you could buy a new one ? or call a mechanic ?
If air is leaking out, obviously the pressure will decrease to nothing. Find the leak and fix it.
You replace it.
If the oil sending unit is leaking oil by the inside of the component, then I don't know of any way to fix that other than removal and replace with another one. If it is leaking by the threads, then you can try teflon tape to seal that. Also make sure it is leaking from the sending unit and not leaking down from the valve cover.
It is probably not the fuel canister itself that is leaking.......on the back side of the canister ther is a small plastic drain valve.....this valve has an O-ring in it that dries up and fails causing the leak. the drain valve has a yellow valve control on it. It is a very cheap part and easy to fix.............Nick
It is possible that the expansion tank on a boiler could have too much water and not enough air, causing leaking. To fix this problem, the tank should be adjusted to the right levels of each.
It is likely the float valve is not seating properly. Needs cleaning or replacement.
Valve cover gaskets?
Replace them. This can be done without removing the heads. Talk to your mechanic about this.
You ask your plumber to instal the valve in your main supply the reduces the pressure but not the flow.