I'm a rebuilder for those Kelsey Hayes 4 piston calipers, Bendix, Budd, disc brake systems, and Mopar suspension parts.
Lets see if i can help you out.
You won't have the unique, special, or factory tools that were invented to get pistons out of their bores, but let me show you what some of them look like to give you an idea in what you might do in getting your stuck piston out.
I would tell you to pump it out hydraulically, by using a grease gun.
You need to block off one threaded end with a bolt and get a grease zerk fitting in the other end of the caliper.
You will need to put back the other piston, and limit it from coming out, by using a "C" clamp, or if its the small half of the caliper housing, put it between the jaws of the vice, and slowly open it up, when the grease forces out the stuck piston.
It's sorta, kinda, messy doing it, but it works.
Kinda hard for me to explain, but use your head, and i think you can figure it out.
Jim V.