Serial number 6099166 would be an early 1958 model.
If the eagle hitch is lowering it is likely the rockshaft piston seal, which is located under the seat, and is contained within the transmission/rearend top plate. Do search here and you'll find a great deal of information on the topic. One caution, Case made a number of different piston/o-ring/seal combinations over the years. Some were designed to replace the original equipment parts. You will need to disassemble the piston before ordering parts/seals/o-rings.