02 VW Passat wagon with brake problem

To make a long story short, after 2 years of tentative repair (I don’t want to list the parts changed so far) the cause for the vibration at highway speed is now within the rear passenger brake not completing disengaging at times. The brake pads were just replaced and the rotor resurfaced, with no out of ordinary anomaly reported. The mechanic now wants to change the master cylinder, but since it is only one of the brake that remain engage I don’t think is a solution.

Does anybody have any suggestion?

Thx.

Part of the VW maintenance schedule is flushing the brake fluid every two years . Has this been done ?

I beleive no flushing was done lately; on the receipt for the new brake pads and rotor resurfacing there was no mentioning for it. But I may add that before the pads replacement every month we had to add brake fluid becouse the dashboard ligth would lit up while no leak was noted anywhere under the car.
Thx