sounds like pump not keeping up. might look like a new one, you might be supprised what a steam clean can do! esecially if you just bought it.
i have a 200tdi i think i could get away with saying new engine just fitted, but it aint never left that engine bay.
not sureabout air lock, as i love puddles of oil on the gravel and have loads, and when my pump squealls because its dry, i just fill the tank up withnew chaep engine oil and it shuts up, power steering works perfect almost straight away, never had air lock, they must self prime, only thing i do is just go lock to lock a couple of times.
as for curing leaks, there isnt no such thing to be honest, but you can hide things for a while, on a defender i have i bought a bottle of bodge slop, cannot remember what it was but from that stand at billing, i diddnt believe a word he told me, but at about £20 for a half litre bottle stumped up tipped in , and you know what, it was cured!, well not really but as its like almost set glue it takes ages to drip out and does last for ages, so i have to say greatly reduces the leak.
getting bored yet!
anyway price of a bottle of bodgeslop times how many bottles over how long will soon equal more than a shiney brand new steering box.
its a bit like tyres 10 sets of remoulds or 1 set of proper tyres.
said ma bit, thanks, and good look with your steering problem.