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Air trapped in the P/s... have you recently replaced the pump? Or turned the wheels left or right with the engine off?
It may bleed itself sitting overnight. I have luck holding the engine at 2500rpm while cranking the wheels stop-to-stop about a dozen times. In extreme cases you can hook up a hand-held mechanical vacuum pump to the reservoir (with an adaptor) and pump 15" of vacuum for 10 minutes.