Power Steering Winding to the Left on Engine Start

I've spent the last couple of days rebuilding the front suspension on my 1965 Jaguar 3.8S (same front suspension set up as a Mk2).

I took out the whole beam to replace the rubber mounting blocks, wishbone bushes, repack the springs and replace the flexible brake hoses (don't get me on to the metric fittings a previous owner butchered onto the calipers that took me a whole day to sort out). It does seem to have sorted the ride height problems. Hooray.

However, on reconnecting the power steering ready for a test run and starting the engine it suddenly wound itself to its limit and takes an effort to fight against it.

You Tube video of a twirling steering wheel and me wrestling it:

I wound the system from lock to lock a couple of times to bleed the system before starting it and since (with engine both running and shut down) - it doesn't have a separate bleed screw.

I had reused the fluid I'd drained as it wasn't that old - perhaps some dirt has entered the system? Would draining it and refilling it with new cure it? (straws to be clutched?)

I had the power steering unit out last summer to rebuild it as it was leaking; it's been fine and continent since then. It was a relatively easy if fiddly and time consuming job and not one I want to repeat!

Something has happened between taking the front beam out and putting it back - I had covered the pipe ends and steering unit unions in an attempt to prevent dirt ingress.

Any ideas?