If the bit inside is free and not gummed up then it is supposed to reset once you have sorted out the rest of the system - ie eqalised the pressure in the two brake circuits. Usually it resets with a bit of help by some sharp pumping of the pedal.
You can sometimes force it to reset by lowering the pressure on the circuit that was previously the higher one, eg by pushing a piston back into the caliper and then giving a sharp jab on the pedal. The pressure in the circuit with the retracted piston will momentarily be lower, while the other circuit will have full pressure. This 'might' cause the sliding valve to recentralise.
Check that the two circuits really are working though. Commonly I find after any brake work on an old system the brake master cylinder seals give up, particularly if you have given some extra long strokes during bleeding. If there is internal leakage in the master cylinder the 2 circuits will not have equal pressures, so the octopus thing will not recentralise.