Anyone experienced their steering is heavier to the right then the left. It also seems that when going round corners the steering does not center itself.

I have had a replacement rack, pump and fluid is ok. Is there anything else that could be causing this?

I have also fitted brand new tyres to the rear and not changed the front 2 as they have around 3-4m left. Can this cause me potential problems?

Any help greatly appreciated

Car is MY93

 

I would suggest you get a full geometry alignment performed; it may be that the rack has not been centred up.
When car runs straight on the road, stop the car, and see how many turns it takes to get it to the end stop, in the left and right direction, if the rack is centred these should be the same.
Self centring is normally down to castor angles.

Power station or a good after market Subaru dealer should be able to tell what is wrong.

 

Yup .. sounds like the geometry is out.
I assume the steering rack was the correc one, was it new or reconditioned and was it fitted by a garage which had tracking gauges etc? or was it a case of screw the track rod ends on the same number of turns that the old one had?

You could also check the tyre pressures (mainly the front ones) just to eliminate that.