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Not being there it is difficult to advise you on how to do the job but there will be some pressure on the hydraulic line. Once the pressure has been relieved, remove and replace hose. There is probably what is called a synchronizing cylinder and there will be no need to try and sync the two cylinders on the slide out. Run it out and hold it a few extra seconds and run it in and do the same will purge the air and it will eventually make it back to the reservoir and if you loose much fluid retract all slideouts and leveling jacks and top of the reservoir with ATF Dextron III.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7