So am I reading this correctly:
1. Air pressure is your PV, Valve position is your CV.
2. Oil is pumped in, which compresses the air above the oil, increasing the pressure in the vessel.
3. The pressure is controlled by venting the air.
Is it fair to assume that there is no air supply, so if you overshoot, you simply have to wait for the oil level to increase? So really, you are just wanting to control the bleed rate at the step when you are adding oil?
And if this is the case, I'm guessing somewhere in this program you write a change to swap the control mode when pulling out oil? (ie E=SP-PV => E=PV-SP by writing a 0 or 1 to word 0, bit 2)