I'll elaborate as you say that this is your first use of PID
I've not worked in the RX3i platform but the manual shows us the following:
The Independent term PID (PID_IND) algorithm calculates the output as: PID Output=Kp*Error+Ki*Error*dt+Kd*Derivative+CVBias
where Kp is the proportional gain, Ki is the integral rate, Kd is the derivative time, and dt is the time interval since the last solution.
The ISA (PID_ISA) algorithm has different coefficients for the terms:
PID Output = Kc * (Error + Error * dt/Ti + Td * Derivative) + CV Bias
This tells us that the equation used is a non-velocity (also could be called positional) form of the equation. Since you're using the independent form I'll use that from now on. Let's forget about I and D and just look at the Proportional component of the output:
PID Output=Kp* Error (Error is either SP-PV or PV-SP, could also be known as direct or reverse acting)
So, in your screenshot Error: 36=1*(500-464). It's working exactly as expected. If your Kp was 10 the output would be 360 for example.
As noted above you need to use more than just proportional to maintain an output with an Error of zero. The integral term is one of those options, which aims to correct error over time or using a gain to add a fixed offset to the output (CVBias). The manual I found online has a block diagram and in it shows the I is calculated as
Integral Term = Previous Integ. Term + Ki * Error * ΔTime
So the integral output references the previous I output - it takes time to wind up and wind down, the bigger the error the faster it winds.
Can you provide more information on your specific application - it will be easier to provide more info then, you mention pump and pressure but there are many pumping applications.
My experience is that is way too much P and not enough I - but like I say it's hard to make an informed comment without knowing more about the application. You may need to use the bias as well.