Abstract: A pressure vessel in the form of a hydro-pneumatic flexible bladder accumulator is provided which includes a shell formed of a cylindrical tube. An open-ended thimble-shaped deformable bladder of resilient material is positioned within the shell. A port plate is attached to one end of the tube, and a closure cap is retained in the other end of the tube. The cap may be retained in the other end of the tube, for example, by a removable locking ring. The closure cap has an inner end which may be displaced radially inwardly from the inner surface of the shell, and it has three axially spaced peripheral grooves on the inner end which respectively receive an O-ring to prevent leakage of liquid out of the accumulator, and two integral beads that surround the mouth of the bladder. One of the integral beads of the bladder has the shape of an O-ring, and it is received in one of the grooves to form an effective seal for the interior of the bladder.