L322 abs fault

Hi, have a td6 vogue se a, registered on 31/10/2006. diesel auto. She started braking left back wheel, then started to only slow down if you pressed the brakes if above 20 miles a hour. Fine if your just moving slowly. Stops no bother, replaced the abs pump and ecu (second hand, guaranteed working). Still just the same, the usual 3 lights lit. And not wanting to stop. Diagnostic machine says left rear speed sensor open. Ordered one, but funny feeling its not the reason for poor brakes, do I have to get the abs unit recalibrated?. 150,000 on the clock, and been relatively trouble free,

Has it been bled properly? If the modulator has been replaced, then normally a diagnostic routine is run to bleed air out the pump and valves in the modulator.