Perhaps this will help. The Thiokol manual for early Imps says use brake fluid type SAE 70R1. DOT 3 brake fluid replaced it and meets or exceeds the SAE 70R1 standard. I suspect the rubber used in the 601 master and slave brake cylinders would not be different from those on an Imp and they might even be the same cylinders. The slave cylinders used on Imps and 1200A Sprtyes (OC4 rear end) are the same.
DOT 4 is a higher boiling point version of DOT 3. Do not use DOT 5 brake fluid as it is not compatible with the rubber used in the those old master and slave cylinders.
Aviation Brake fluid and Aviation Hydraulic Fluid are both rated to Minus 90. That's what we replace all the factory stuff with in the man lifts, and trucks, fork lifts, loaders,...And Snow Cat's, for use "up here",.... 350 miles North of the arctic circle where we see ambient temperatures to Minus 60. Works fine.
I thought I saw some where about the steering systems in the sprites and 601's using hydraulic fluid. I cant find it any place appears it was only the 2100 which uses hydraulic pressure to actuate the steering cylinders.
Different types of aviation brake fluids are available. Some times are compatible with rubber the same way DOT 3 and 4 are, and others are compatible with non-rubber (neoprene) and dissolve rubber in the same way DOT 5 does. Pick wisely -