Unloaders are not pressure regulators. They should not be used for setting pressure. They are there only to prevent the pump from hydraulically exploding from trying to pumps into a confined space.
If you want to lower the pressure, you should use a larger orifice nozzle in your wand.
It looks like this unloader is the cheaper kind that has no buffer tank bypass output, it only recirculates to the pump inlet. That's why you do not want to let the pressure washer run in "unloaded" state for any prolonged period-- you will burn up the pump because all that shaft work heat into the water has nowhere to go.
That's why pro washers use buffer/snubber tanks so the unloader valve flow recirculates in a much higher volume loop with more water and more surface area. This keeps the pump cool when in prolonged bypass.
You probably know that pumps don't make pressure, they produce flow. The restriction of the nozzle is what causes the pressure. So you can vary the pressure with the nozzle.
With my 4.0gpm washer, the 5.0 nozzle was the sweet spot, it runs about 2300 psi. Cleans well and is SUPER FAST. The 4.5 will run about 2500psi. It wasn't worth the loss of flow (minor as it was) to run only a tiny bit more pressure.
If you look at a pw orifice chart, you'll figure our pretty soon that the orifices are calibrated in GPM at 4000psi. So a #5 will give 4000psi at 5 GPM. A #6 will give 4000psi at 6gpm, etc.
So unless you need 4000psi (you don't), you should have the nozzle number at least one step larger than your rated pump flow to drop the pressure down to a more useful 2500-3000psi range.
