i have an omega mv6 but it has a problem with the power steering going heavy when i try to park it!!!.only when engines on tick over. took it to vauxhall and they said"nothing wrong with it apart from when you park it"!!!! idiots or what???
Could be the pump on the way out......try giving a few highish revs as you turn the wheels lock to lock then again with engine at tickover.......if it's the pump then you should find it easier at higher revs
Is the pump belt tensioned correctly?
Perhaps there is air in the system (unlikely if not having to top up)
Only other thing I can suggest is to drain the fluid, refill with fresh to correct level, turning rack slowly lock to lock to expel any air