Old thread, but recently got hoses and ag fittings from discount hydraulic supply… the exact same problem. Replaced with another brand and no problem. You would have thought by now they would improve the quality of what they are selling.
I know this is an old thread, but I have a 1996 5500 with this issue as well. Had it to the dealer and they replaced the pump, the hydraulic filter unit that includes the check valve, as well as replaced the high pressure relief valve.
I changed fluid and tried low viscosity as well. But if...