Kelsey Hayes 4 piston calipers help

You use a grease gun, and pump them out hydraulically.
You need to fabricate something up, that will screw into one of the hose threaded ports, and block off the other port.
You will need to use "C" clamps to limit the travel of the pistons, as they will start to come out of their bores unevenly, or use a block of wood in a vice, and then slowly open the vice as your pumping in the grease, and the pistons start to come out.
Use your imagination, and you can figure it out.
Here's some tools that were available, back in the day, to get the job done.
Good luck.
Jim V.