Really stiff power steering when turning slow

[Jamesey]

Hi im new to this forum and I've had for just over a year now a 2009 V70 r design 2.0d the power steering has got worse over the year but so far I've had the fluid flushed out and replaced 3 times , had a new top up bottle fitted due to the filter in there and had a 2nd hand pump fitted .
And when reversing off my drive and turning around slow the steering is really really heavy but as it's stiff I can hear pressure like you can hear when it's on full lock which is strange .
When I go on a drive the steering becomes as light as a feather too light if anything .
I'm trying to eliminate everything before I bite the bullet and replace the rack , my 18" 245 40 18s are at 40 psi so not them .
That pressure swishing noise when it's stiff is what the problem is .
I've been under the car and it's a straight forward hydraulic rack with no sensors or wires anywhere , I've no settings in my control panel for soft med high steering ,
I'm at a bit of a loss really
Any help and experiences would be swell , thanks