This valve - an Apex 3.7m PRV around 5 years old - has been dripping on and off for some time but yesterday it started gushing. I pulled the valve apart but there doesn't seem to be anything wrong with the spring or the valve seat. Maybe the spring gets tired over time?

Ok, I can buy a new valve for about $130 but it seems there should be repair kits available. I mean, the only working parts are a spring and a plastic non-return valve.

Anyone have any tips on this?

BTW, I've backed off the Apex feed valve about 5 full turns and the hot water is running from the taps but only weakly. The PRV doesn't drip at this pressure. The valve is situated on the roof of a single story house, about 3.5m above the low pressure cylinder. The cylinder, feed valve and pressure release valve were all replaced at the same time, about five years back.