Do a search on this site, and on RV Net site for hydraulic pump leak. I went through this a couple years ago, and just replacing the O-ring will not cure the problem. Remove the plastic tank from pump, clean inside with break cleaner. Get new oring, if yours dont have any nicks it can be reused! Install oring, put a bead of the black silicone sealer around the outer edge of oring, and smear some around the tank ahead of the oring. Slide the tank back into place, being careful not to move the oring. LET SET FOR AT LEAST 24HRS.!!! (minimum) Finish putting everything back together, and make some kid of support to put under the end of the oil tank! Refill with the proper oil and run slides in and out at least 3 times to get air out of the lines.
Rod & DeeDee Wise
04 Everest 343L, 07 Chevy 3500 SRW,4x4,D/A,ext.cab