A pressure limiter (10) for use in a downhole tool having a positive displacement pump and an inflatable packer, includes an outer housing (42) with an inner mandrel (62) disposed therein. Slidably disposed between the mandrel (62) and housing (42) is a generally annular piston (110). A spring (68) biases the piston (110) toward a closed position (as shown). Packer pressure acts upwardly on the piston tending to move it to an open position in which pump discharge pressure is relieved to a well annulus. A check valve (142) prevents premature relief of pressure of the packer. Once the piston is in an open position, packer pressure will maintain it in the open position until pressure is relieved from the packer.
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