Powermax HE Low pressure/High pressure

OK, so the main expansion vessel inside the unit needs pumping back up to 1.0 bar?

I've read differing things on the internet.

I plan to drain the system fully, so do I test the expansion vessel Schraeder valve when completely empty or should I do it when the system is full of water to see if water comes out?

So do I have to have the system drained and the drain cock OPEN in order to pump it back up and set the pressure again?

We have a secondary expansion vessel in the attic. I tested the pressure on that (whilst system contained water) and it came back at 26psi. The manual says it should be 36-40 psi. Again, recharge both of these when the system is fully empty and the drain cock is open?

Sometimes the digital readout goes as far as 3.4 bar (it does eventually come down again). Does this indicate there's a fault with the PRV? Shouldn't the PRV 'burst' at 3.0 bar and therefore never allow the pressure reading to go over 3.0? Or does the fact that it comes down indicate that the PRV is allowing pressure out?

Thanks.