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.

On 7/6/2014, Rob Zucca wrote:
"I replaced the master and went through the hydraulics replacing the hoses, flushing etc. I bought the TRW from Moss. They have not corrected the residual pressure issue. I sent Moss an email regarding this about three months ago and have not had a reply. I corrected it with the 1/32 hole. I assumed the issue had been corrected and went ahead and installed it. Then noticed the rotors dragging after bleeding. Sure would have been easier to fix BEFORE I installed it". -- Rob

On Jul 14, 2014, at 11:57 AM, "Kelvin Dodd" of MossMotors wrote:
The brake master cylinder is more of a problem. Legally, if we open the packaging, we are responsible for all liability for any possible brake failure. As you can understand, that is not a position we can put ourselves into.
I'm going to inspect some of the current stock and get an idea as to whether TRW is either drilling the return valve, or have changed the design to one that offers less resistance to return pressure. My understanding is that all of the cylinders now had the drilled valve, but evidently from your experience that is not correct.
If the samples are still holding pressure, I will have to figure out the next move. This is a high demand item, so blocking sales is going to be an issue if we have no other solution. We do carry the AP/Caparo manufactured unit, but I've no idea of it's internal construction and do not know what holding pressure it may have. Right now, I do not have the ability to pressure test master cylinders as the equipment is not set up".

Once again the answer is lip service but no immediate solution, and no commitment to either stop sales of the faulty part or to assure that it would be fixed any time in the foreseeable future. Let the buyer beware.