Nothing short of rigging up a test bench which would cost a couple of hundred dollars at a minimum.

Hydraulic pump, suitable hoses, piston retainers (could use a block of wood) and a gauge. This would only test total pressure. Would work only on a single caliper design. Wouldn't tell whether the pressure is applied evenly to multiple calipers.

Initial test on a new rebuild uses vacuum but this method can't be trusted on a used caliper due to possible wear on the piston.

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