Power steering failure... Again...

Hi to you all.
Some help needed with diagnosing power steering issues.

It all started with a leaking pas pump. Ordered an overhaul kit, took the pump out, replaced the seals and gasket, built it back in and had a well functioning steering system for a short while.
Unfortunately the gasket blew (probably faced it the wrong way). I ordered a new pump, built it in, adjusted the steering box to get rid of the excessive slack and enjoyed at least another day of driving with a proper functioning steering system.
Not long after, I almost completely lost PAS. While driving it's fine of course, but low speed manoeuvring is almost impossible.
The new oil I put in looks black and polluted, not foamy however.
No sign of leaks.
I'm now thinking direction steering box, but would rather avoid replacing it if not necessary (LHD.

She's a 2003 td5 Defender.

Any help will be greatly appreciated.

I have to add that when I rev up (more pressure) the steering becomes a bit better. Could it by some miserable coincidence be the new pump?

Difficult to say, which probably explains the lack of responses you're getting. If you put the seals in wrong then it could be user error? Did you bleed it? It might be best have a mechanic check and/or install the pump and see if the problem still presents itself.

Hope you did not over tighten the steering box adjuster. Adjustment has to be done only with wheels/pump arm set straight ahead and allan key bolt only turned until contact just felt. Any slack with wheels turned to be ignored.

Everything felt fine after adjusting. Both wheels of the ground, free turning left and right with no hard points.
I know that too tight can lead to damage, but not likely to lose pressure? Or?