Caliper manufacturers and knowledgeable racing mechanics recommend against splitting the caliper halves when rebuilding. If you do split the caliper, the cross feed seal must be replaced prior to reassembly.
This square-section O-ring seal measures 0.500" OD x 0.110" thick. Fits many AP Racing / Lockheed calipers including the common LD19 and LD20 calipers used on small formula cars and sports racers. Sold individually.