I have the OPV rebuild kit from Stefano. The kit consists of a spring, a tube of silicone grease, an o-ring, and a silicone valve seal. I took the adjusting screw, spring, and piston from the valve body, and discarded the old spring, o-ring, and valve seal.

How thoroughly do the removed brass parts need to be cleaned before reassembly? I've scrubbed them with hot soapy water and a toothbrush. Is that sufficient?

Where do I put the silicone grease when reassembling the valve, and how much? Just a thin coat on the o-ring, or does the silicone seat or the piston need some grease too?

I know these are pretty basic questions, but I'd like to make sure I get it right the first time so I don't have to take it apart again.

I just put a light coating of silicone grease on the o-rings and on the threads. It adjusts very smoothly now. It didn't reduce the high rate of bypass flow, however. Perhaps the pump and motor are just too strong.

What kind of pump? If it's a rotary pump as your profile notes, then you should adjust it via the bypass valve on the pump, not via an over-pressure valve like a vibratory pump. With a rotary pump espresso machine, the over-pressure valve is typically set to open at 12 bar and the pump's bypass valve is set to 9 bar. Under normal operation of a rotary pump, the OPV passes no water; it only allows water to pass due to water expansion when the pump isn't running.

Yes, it's the rotary pump machine in my profile. It's a Procon pump. Where is the bypass valve on the pump? I see a large cap nut on the intake side. Is there a valve in there, perhaps?

See "pressure adjusting screw" in How to adjust brew pressure of rotary pump espresso machine?

Thank you, Dan. Since my OPV, which is in the output line between the pump and the boiler, is flowing water even when there is no back pressure from brewing (i.e., portafilter is empty or not present), then does that suggest that the pressure coming from my pump is too high? I should turn the pressure adjusting screw counter-clockwise to decrease the pressure?

Set the pump pressure to 9 bar using the pump's adjustment screw as instructed here. Then adjust the OPV such that no water flows at that pressure.

I've read that thread and the other thread linked to from the post you directed me to. I'm unsure of the process for adjusting the screw. When I start to remove the cap nut that covers the pressure adjusting screw while the line is under pressure, water sprays out. Is that normal, or does that indicate that something is wrong with the pump? Is it necessary to cut the water supply, adjust the screw, restore the supply, then test the pressure and repeat that incrementally?

What is the inlet pressure? Maybe the incoming pressure is too high. Keep in mind that a rotary pump can only increase pressure, it can't decrease it. If the adjustment screw is to the point that it's unscrewing completely, that means the relief valve is returning 100% of the incoming water and still not reducing pressure. Either the inlet pressure is too high, or the relief valve is stuck (and thus the adjustment screw does nothing).

My household water line pressure is about 60 PSI, but I've got a regulator on the supply line that goes to a water softener and then to the espresso machine. It is set to 20 psi.