I think it would be unusual (though possible) for the master cyl. you replaced a year ago to have failed. When you bleed your brakes, are you getting clear fluid at a strong flow through all four bleeders? Are you confident your bleeding procedure is clearing air from the lines and not reintroducing it? Is your fluid level dropping, possibly into the vacuum unit? Do you know if the fluid has been changed on a regular interval? My own '88 hadn't had its fluid changed frequently enough, resulting in a failed ABS unit. In my case that was evidenced by a poor, intermitent flow to the rear calipers. Just a few thoughts.