Any ideas what could cause the power steering reservoir, to completely drain itself

I thought the rough steering was due to the tyres I had fitted, as the problem only apeared after they were on the car.

Ive looked into the reservoir this morning and its bone dry VX say its knackered, but Ive just topped up with over half a litre of there own fluid, and it seems ok, slight leak where Ive split some fluid as its stupidly difficult to get into, the car now steers around corners without me dislocating my shoulders.

Im giving it a 100mile trip in an hour as I need to collect some goods, so Im guessing any of the spilt fluid should have dissapeared by then

VX also state that the reservoir should never need topping up..... if thats the case, why fit a max and min level line on it