Hydraulic jacks use hydraulic pressure to lift loads that normal screw-type jacks simply can't lift. If people use the jack incorrectly or the hydraulic oil somehow leaks out of the device, the jack can lose its prime and begin to feel soft or spongy while you're using it. Priming the hydraulic jack restores it to full function. While several types of hydraulic jacks exist, you prime them all using the same basic technique.
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Close the release valve on the jack.
Raise the jack's piston out of the hydraulic cylinder. Pump the jack's lever to raise the piston.
Open the jack's release valve while the piston is raised.
Lower the piston back into the hydraulic cylinder. The motion forces oil through the inner workings of the jack.
Close the release valve and pump the lever again. If the jack still feels soft or spongy, repeat the process until the device is fully primed.