What I'm trying to do is get the Z axis (syringe driver) to withdraw completely, to the point where it trips a switch telling the axis to stop moving, in order to ensure that has sufficient clearance to go and pick up the next 'syringe' of paint.
When it has done this, the Z drive moves down until it contacts the back of the 'syringe' plunger, then stops moving.
Currently this last operation is achieved using homing. I was hoping to achieve the 'withdrawal' stage using G31, a macro, a hack, whatever, in veiw of the fact that homing the same axis in 2 directions is not allowed in Mach.