ACFT EXPERIENCES WAKE TURB DURING STEEP DSCNT INTO SFO.

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BAY PUT US ON A BASE LEG AT 6000 FT 8 MI FROM RWY 19L. I ASKED IF THIS WAS VECTORS TO ILS ('YES') AND WAS HE GOING TO PUT US ON THE GS ('MAYBE'). THEN HE ASKED IF WE COULD GET DOWN AND OFFERED TO BREAK US OUT FOR IT WOULD EITHER BE THIS SLAM-DUNK TO THE ILS OR A 20 MI FINAL. WE ACCEPTED THE SLAM-DUNK. IT WAS A MISTAKE. WE HAD TO DIVE TO MAKE IT. AT ABOUT 4 MI FINAL WE ENCOUNTERED SERIOUS WAKE TURB FROM A B757 4 MI AHEAD THAT THE CTLR DIDN'T BOTHER TO TELL US ABOUT UNTIL WE WERE CLRED FOR APCH. BAY DOES THIS FREQUENTLY; IE; FORGETS TO TELL ABOUT A WAKE GENERATOR. ALSO; THEY FREQUENTLY PUT YOU ON THE ILS WAY ABOVE THE GS. THAT IS ILLEGAL AND UNSAFE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR PARTICIPATED IN THE WAKE TURB STRUCTURED CALLBACK PROGRAM. RPTR FEELS THAT TRACON INDUCED THE SIT BY GIVING A HIGH STEEP APCH TO THEM WITH LITTLE SEPARATION FROM PROCEEDING ACFT. IT WAS NOT KNOWN TO FLC THAT THE ACFT THEY WERE FOLLOWING WAS A B757 UNTIL THEY WERE ON TWR FREQ. BECAUSE THE APCH WAS CLOSE IN; IT FORCED THEM TO BE STEEP WITH AN IDLE PWR SETTING. THIS MEANT THAT THEY DIDN'T HAVE A STABILIZED PWR SETTING WHICH WOULD HELP THEM RECOVER QUICKLY IF THEY SHOULD HAVE AN UPSET SIT. EVEN WITH A HIGH STEEP DSCNT; THE RPTR ENCOUNTERED WAKE TURB. THE TURB WASN'T TOO SEVERE AND ONLY LASTED 3-4 SECONDS.