Sounds like when I had fecked pistons. They corrode underneath the seals and then it sticks a bit sometimes, or others it eats the seals then the piss oil everywhere.
New seal kit and polish up the pistons should solve it. Just hold the inside bit with the magnet in the chuck of a hand drill, put a rag on the edge of a table or workbench with some Autosol on it and whizz it round until they're nice and shiny, clean then thoroughly, lube them with brake fluid, rebuild the brake with new seals and rebleed it, should sort it out.