There has been a recent run of a particular fault in a new replacement part master cylinder. The part has finally been identified as TRW/Lucas dual bore master cylinder with 7/8 inch bore for MGA (also for Austin Healey Sprite, MG Midget through September 1966).

The problem is that the brake hydraulic system will release most of the pressure so you can drive the car, but it retains enough residual pressure to make the brakes drag and get hot while driving. This fault may be related to a single production run of the master cylinders, not necessarily affecting all parts form the same manufacturer.

Meanwhile there is a way to fix it, so you don't have to buy another new master cylinder. See BT-101A for more information.