Carver Mk II Vacuum Release Valve.
The Vacuum/pressure release valve seen above, is shown as taken out of the body assembly, which is screwed into the Water heater.
To improve the sealing properties and stop leaks from this point, expand the set length of the spring by about 0.25 inches.
The spring and piston with is located inside the main screwed body, then is secured with the right hand device which has a flat screw head. It is the spring alone which provides the seal.
Note: In recent correspondence, Gary has recommended that the modification is not needed as it is too strong in the first place. Attention should be given to the Pump pressure, i.e. reducing it to eliminate leaks at this point.