The front calipers are really inexpensive (<$30) from almost any auto parts store and generally aren't worth rebuilding.
Napa # SDC2421008 for the right front (87 Eagle Wagon)
Napa # SDC2421009 for the left front (87 Eagle Wagon) $13.00 Each
AMG 05579865 Front Caliper Rebuild Kit (2-calipers, no bleeder) $16.82-
AMG 05744403 Rear Caliper Rebuild Kit (1-caliper no bleeder) $15.62-
AMG 05578802 Caliper Bleeder Valve $0.65-
AMG 05597172 Caliper Bleeder Valve Cap $0.48-
AMG 05744405 Rear Caliper Rebuild Kit $37.80
Cardone 184181/184182. Unloaded Front calipers, reman. Metal Piston.
Cardone 154181/154182 Front Caliper with Severe Duty pad, reman. Metal Piston
Cardone 164181/164182 Front Caliper with standard duty pad, reman. Metal Piston
Raybestos FRC4147/FRC4148 or RC4147/RC4148 Front Caliper with pad. Reman.
Bendix 55266/55267 Front Caliper Reman.
The original rears are also fairly cheap unless you have the dual pull Wildwood calipers. Here's a posting about them:
http://www.hummerknowledgebase.com/brakes/rebuilding_rear_calipers.pdf