A radial type hydraulic pump motor according to the present invention has pistons, each of which is formed at its head portion with a concave or convex spherical surface. Engaged spherically with said spherical surface head portion is a piston shoe whose outer planar portion is slidably engaged with the inner planar portion of a crescent-shaped segment arcuately engaged with the inner peripheral surface of a drum case. Said piston shoe is formed at its outer planar portion with a hydraulic recess equal to the effective pressure receiving area of piston in area. Said recess is maintained in communication with a cylinder chamber. Accordingly, said spherically engaging portion and arcuately engaging portion make little sliding movements and only the outer planar portion of piston shoe and the inner planar portion of segment mutually make sliding movement under little contact pressure, thereby causing little wear to said engaging portion and sliding portion.
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