Home in the in garage, I found not only the power steering pump to be leaking oil, but all the lines and the steering rack. I cleaned everything, replaced the old brittle return hoses to the oil cooler and refilled with Redline power steering oil - this time only up to the bottom of the dipstick, to allow capacity for the fluid to expand. Next track day, the power steering was still leaking. The power steering pump is simply overwhelmed by the high RPMs, it builds too much heat and pressure and cavitates the oil.
As I enjoy the thought of interfacing new electronics to the car, I decided to add an electric power steering pump to resolve the issue. After some research, I found that the TRW/Vauxhall Astra H power steering pump is readily available second hand at a low price - and only needs power and an on/off signal. Everything is contained in one unit - pump, controller, reservoir. It regulates power based on demands. It supports an idle state, and a running state (separate 12V signals).
The pump is connected to the steering rack using high pressure AN6 fittings and nylon braided PE hose rated for 2500 psi. The pump is powered directly from the battery with an inline 100A ANL-fuse.
The pump is temporarily mounted using a heavy duty perforated fixing band, which is fixed to the frame with M6 bolts and inserts.. My plan is to make a CNC machined bracket which also suspends the pump on rubber isolators, as it was in the donor car. As of now, it sounds like a jet engine starting when I turn on the on the ignition. I will also add a switch to cut the power supply when the car is stationary and power steering is not needed, as it can quickly drain the battery without the engine running.
The pump is designed to use Pentosin CHF 11S or similar spec fluid, green color smelling strange. I drained the power steering fluid from the steering rack and oil cooler using (low pressure) compressed air, then refilled the circuit using the CHF fluid. After 3 years, the CHF fluid does not appear to cause any issues with the seals or performance of the steering rack.
Without the stock power steering pump, I had to reroute and find a shorter belt. After a few trials, I found that a 6PK1885 belt worked well.
Old power steering pump and plumbing now in the parts bin.
The electric power steering performance is as good as OEM. The steering is slightly heavier, which is only an added bonus. There are no noticeable delays and the driving feels just as intuitive as the stock setup. Also, there are no more oil leaks on track days, even the power steering rack stays dry. My guess is that the electric power steering doesn't build as much heat and pressure as the stock setup, as it runs independent of engine RPM.