Clark C500 having hydraulic issues!!!!

My Clark C500 HY 70 is having major hydraulic problems. I just restored the truck that had been sitting up for more than a year. However, I did not do anything to the hydraulic system at all. I was using the truck as normal & noticed a sudden strain on the engine & lost function of my hydraulics. It's almost as tho the engine is strained bc the hydraulic system is asking for pressure all the time. The fluid was very milky so I pumped the fluid out, cleaned the tank, changed the hydraulic filter in the tank, added all new oil & 1 gallon off lucas hydraulic conditioner. When I cranked it back up the engine still sounds strained. I was able to rev the engine all the way & get the hydraulics to work but it's obviously pressurizing my tank bc it blew the breather/dipstick out & covered the truck with oil. I'm thinking I either have a stuck pressure relief valve, plugged return hose, or stuck part in my valve body. Any help would be much appreciated.