1993 Mazda MPV brakes

hi the brake pedal would slowly go to the floor so i replaced the master cylinder but after much bleeding of the brakes,still somewhat soft but serviceable now the rear wheels will lock when cold,checked the shoes appear clean no oil also approx 75% left on linings so adjusted rear shoes improved for a while again brakes very eratic ,could there be a abs problem if so recomendations would be much appretiated gary