My 64 had PS fluid that looked so so when I bought the old girl and I ran it the first summer brush cutting while monitoring all other fliuids after changing.

Kind of a shake-out and the PS was the last thing addressed after the first 200-300 hours. Usual leaks around the pto engagement shaft and some on the PS lines were found; be careful re-tightening any of these original fittings and check that the pipe threads into the source are not the culprit, vs. the tubing fittings as all can leak from age and vibration over time on these 40-50 year old units.

With my unit well warmed from use, I dropped the PS fluid from the drain under the front bolster using a clean white platic drain pan. Fluid actually had what looked like mud and I let it drain for over an hour to get as much out as possible. I then squirted about 1/2 a quart of Valvoline Max Life DEX/MERC over the top of the gear in the front housing, through the top access hole in an attepmt to get more of the crud removed. Refilled with the DEX/MERC until the gear was visibly covered, I then worked the steering wheel back and forth repeatedly only to find I had no PS. Working the wheel back and forth about ten minutes more (with the vent cap off) started to bring it back. I then scraped all the crud off of the two dimpled recess vents on the side of the tall cap as they were plugged solid. I assume this cap is filled with steel wool like the trans cap and I wanted to get the venting capacity back if possible. Dropped the fluid again after a 1/2 hour of tractor use, turning the steering wheel back and forth while driving and could see some more crud had been freed up. Drained well and re-filled with DEX/MERC; it was not until I ran the tractor for a good hour before feeling I had full ease of steering again. Make sure to add fluid to cover the gear the appropriate amount and I used a screw driver to check same after turning the steering wheel until you get the gear where you can do this through the fill hole on the bolster.  PS on these units is great and mine now operates very smooth again and can be turned with very little effort. I add spinners to my tractors as it makes driving these D15II units with PS almost effortless.

To expand on what Tim says, I would recommend taking the front support apart to flush out whatever sludge or other dirt is in there since there is no filter. I was surprised at how much crud was in my D17's.