Volvo Repair: Intermittent severe brake pedel chatter, volvo 850 wagon, volvo specialist

Question
97 volvo 850 Wagon T5
Once every day or two during slow speed braking, the pedel chatters severly for a second or two.
Over the last 3 years my regular mechanic (not a Volvo specialist) has replaced rotors, pads, the ABS pump (used from wrecking yard), and the master cylinder. He also added some DOT 3 brake fluid instead of DOT 4 about  3 years ago but the brakes have been flushed several times since then. My mechanic used rabestos pads on the front and OEM pads on the rear.
Thank You,
Roy

Answer
sounds like an abs problem but without seeign it I can not be sure.most common problem is the ABS  control units go bad.it would need to be checked for diagnostic trouble codes with a scan tool.the other brake vibration sometimes that happens is a loud resonating sound which can be fro the rear pads not having the shims installed.there are shims for the pads that can eliminate this sound and they do not come with the pads