Abstract: A deformable sealing member, particularly a sealing tube, which is at least partially elastic is secured at two distant locations to two working members, e.g. to a piston part and a cylinder part respectively to form a leakproof working chamber for pressurizing therein a working fluid or for transferring the power of a pressurized working fluid introduced in the working chamber to said working members, the apparatus thus being able to operate as a pump or motor respectively. The sealing member is supported against the working pressure in said chamber by a support surface, and between said sealing member and said support surface there being a low-friction component which is constituted by a solid material, e.g.