A hand device for varying the gas pressure in a valve-less ball comprises a holding structure in which the ball is held non-rotatably and an injection syringe which is introduced into a longitudinal bore in a handle of the holding structure. During the introduction movement the hollow needle of the syringe perforates the wall of the ball and finally projects by a portion thereof into the interior of the ball. The syringe can be operated to inject into the ball a sealing agent which has been previously drawn into the injection syringe and then, after the piston has been removed from the syringe, the connecting portion of an air pump may be introduced into the open end of the syringe cylinder to pump the ball up to the desired pressure which is indicated by a pressure measuring arrangement, the injected sealing agent closing off the perforation formed by the needle when the syringe and the air pump are withdrawn from the holding structure.