Abstract: A rotary hydraulic device that includes a housing with support plates mounted against rotation. A pair of pressure plates are mounted on the support plates and cooperate with a surrounding cam ring to form a rotor cavity. A rotor is disposed for rotation with the rotor cavity, and has vanes that radially engage the surrounding surface of the cam ring. A circumferentially continuous hydrostatic pressure pool is formed between each pressure plate and its adjacent support plate for balancing and/or slightly exceeding the forces in the pump cavities that tend to separate the pressure plates. An isolated area within each hydrostatic pressure pool intermittently communicates with the pumping chambers through timing passages in the rotor. Fluid flowing to this isolated area may be employed to form a supplemental hydrostatic pressure pool for enhanced axial balance on the pressure plates, and/or for directing discharged flow through multistaged orifices to precompress the fluid volume to be displaced.