After adjusting the air output of the main gyro I switched my attention to the directional rate gyro. I had noticed that the gyro was slow in spinning up and needed a lot of air to get going. I noticed some paper flapping about on my desk and so discovered that there was quite a lot of air leaking out of the gyro which I traced to a badly worn seal ring in the gyro mount. Fortunately I still had some spare bits which included the seal ring and holder so a repair was duly made. As a result I can now run the main and directional rate gyros together at reasonable pressure from my compressor and play around with the directional control system as a whole. I found I had to adjust the rate gyro return spring slightly as it had a bit of bias to one side, after that the system is starting to perform as I would expect.
The rate gyro accelarates the movement of the rudder servo while the gyro platform turns away from the directional setpoint; at the end of the motion you see the piston return slightly. The rudder will now turn the V1 back towards the setpoint, during this motion the signal from the rate gyro opposes that of the main gyro, ensuring that the V1 turns back to the setpoint slowly and smoothly.
You can see the directional control at work here: https://youtu.be/ntO-3VWGbrQ