Here's another slant; via Chicagoland MG Club's site that indicates that it's the Master Cylinder where the problem lies, not at the wheel.
"The Factory Workshop Manual gives explicit instructions on setting the master cylinder pushrod, which sometimes cures the problem. However, even with the proper 'free play' in the pedal, the brakes can still bind up. The problem is that the master cylinder piston is not coming far enough forward to uncover the bleeder orifice that allows the expanded fluid to bleed into the reservoir."
"Solution: Add a shim between the master cylinder block and the cover plate! This shim allows the piston to come a little further forward, thus uncovering the bleeder orifice"
They go on to suggest the shim size, etc.
Never had this issue before, but what do you think?