Yes, I had them mixed up, as the diagram I was looking at was unclear about the direction of water flow. The first valve someone has marked as a pump priming valve.

My machine has an additional valve, an adjustable OPV directly connected to the pump.

Each one is what it is.
If it's between the closed valves (check and brew) while the machine heats then it IS the expansion valve. If it's the ONLY OPV on the brew circuit then the expansion valve is the only valve you've got for adjusting maximum brew pressure. If the machine also has an adjustable OPV to allow for a settable maximum brew pressure, then that's great, that one will be before the check valve where it gets to lead a luxurious life without being aged and scalded with boiling water, in it's nice cool location it will last much longer, not build up calcium deposits and hopefully remain adjustable and precice for years. That's why good manufacturers of good machines bother to have both.

EDIT: This was posted before I read that the machine may not have an OPV before the check valve, if it even has a check valve other than the Ulka's. If your machine does have only one OPV, then it can be adjusted down to 10 bar or below if that's how you want to control your maximum brew pressure.

Thank you for the replies. I understand the purpose of the two valves. The one near the pump is a 12 bar overall safety valve. The valve closer to the boiler is an OPV (though technically not adjustable on my machine). My original question has to do with running the machine with a blind basket. My pressure holds steady at 12 bar while the pump continues to vibrate, but I don't see any water flowing back into the reservoir. Shouldn't a valve somewhere open up to relieve some pressure?

Here is the page from the parts manual showing the water flow and the valves.

If I correctly understand that diagram. if the pump is able to increase the pressure past the OPV valve pressure, water should flow back into the reservoir.

Thanks for the diagram... That's kinda cool, Technically "Assembly 11" is a check valve and an OPV. Parts 2 and 3 are the brew circuit's check valve ensuring that all the water can flow into and through that valve toward the Heat Exchanger, but never allowing HX water to flow back toward the pump. Because the OPV parts 7, 8 and 4 are downstream of the check valve, they're an expansion valve. They will limit the machine's maximum pressure to 12 Bar. I agree with the opinion that part 15 (nearer the pump) is a priming valve, not an OPV. It is actually designed to relieve at LOW pressure, like a vacuum breaker, so as to let out air from the pump and let the pump prime, then it closes once the pressure is enough to close it.

The Expansion Valve side of the assembly 11 does look adjustable in the picture. I don't see any harm in adjusting it down to 10 bar to see how it performs, maybe lower. Just be sure that it doesn't constantly leak, it should only relieve while the element is on expanding the water, if it has a constant leakage it will allow the HX to boil dry.

If you search on the part number for assembly 11 (5965212AL) you will see that some suppliers specifically name it as adjustable from 9-12 bars.

With a blind basket, at some point adjusting it down you should see water flowing (more like dribbling) back into the reservoir.

If you do not see that happening currently, then either:

Your gauge is not calibrated properly, and the opv is applying > the 12 bar spec pressure, and over the max pressure of the pump.

Your pump is no longer able to pump over 12 bar, and the gauge is reading properly.

The pump is applying over 12 bar pressure, your gauge is reading properly, and you are just not noticing the water being returned to the reservoir.

In all of these cases, you should notice a small amount of water being returned to the reservoir through part 15 as pressure builds up, and until that valve closes. At least, that is what I see, with what I think is a similar valve in my bz99.