My PSP must have had a bad leak, not much fluid left in it when I removed it from vehicle.

Replaced it with another from wreck. Drained fluid prior to fitting, might have had small amount of water in the pump as the top was missing and bonnet from wreck left open.

Fitted PSP, topped up with fluid, turning wheel to bleed system was very hard. Gave the system a hard rev, heard something scream, checked vehicle all ok, now PSP must be working as wheel turned easily ..done full L & R wheel lock x 3, checked PSP fluid hmmmm, looked milky, went for test drive, no leaks seen (as yet) PSP fluid looks less milky but not as clean as what I put in.

Just wondering whether I need to drain all PSP Fluid and replenish with new again, or will it work as is?

Yes.

Is it milky or is it bubbles? If it was bubbles then its just the air getting bled back through. However its probably a good time to drain and replace anyway

Ok,

Might have been bit of both.

Question is How do I drain all the fluid without causing PSP to go dry?

Can I undo the return line. i.e. the rubber hose that's not the high pressure hose and place a hose on the steel pipe to run fluid directly to a large container with motor running?