8011 veers to right

I have a Zetor 8011 and when I drive it, it veers to to the right.
To try to find the problem I have;
- checked toe-in (this is 6mm)
- replaced steering oil filter and oil.
- switched rear wheels.
- switched front wheels.
- checked front bearings.
- measured left and right between rear and front axle, no difference found.
- checked brakes for dragging, they run free.

What could be wrong?

Thanks

Are the rear tyres the same diameter?

how bad is it ?

if you jack the front end up and start the engine does the steering stay put or do the front wheels creep one way.

our 6711 will turn itself to full right lock unless the steering wheel is held still. i guess something adjustable in the power steering linkage or a leak, perhaps a cylinder reseal would cure it , but its liivable with .

a neighbour had a leyland with power steering that needed the steering wheel slowly rotated to the left to keep it in a straight line on the road....odd at first but after a while you compensate without thinking about it.