I've taken them apart, it isn't difficult. Nothing there but a spring and a plunger. There is a shim or shims behind the spring to adjust where the plunger sits when it is not under pressure. One would think you could effectively stiffen the spring by adding shims. However, I remember Mick Hemmings commenting in his DVD that he thought this was a poor idea as he wanted to be able to pick up the valve and hear the plunger rattle when he shook it. What I don't remember is whether he suggested a stiffer spring to increase pressure. Nor do I remember why he wanted it to rattle, but I respect that he had a reason. I have never run an oil pressure gauge, but if I had one, I would be measuring the resistance to compression of the spring and finding springs of different values and testing to see what it did. I'd be interested to know if anyone has done this.