removal of plunger from injector pump on a zetor 5911

Does anyone know how to remove the piston/plunger from the pump body, in order to replace a broken spring ?. I thought that once undoing the small screw on the clamp that it might then slide upward with gentle pressure from below, BUT it would appear not !!, any help here would be greatly appreciated as I need to get the old girl up and running asap.

I would scratch mark the exact timing before taking it loose !! Or else it will run in disbalance. you can rotate the plungers in the pump body, thereby timing each cylinder.