Whilst carrying out a pre-MOT on our ERA (R1031). I was annoyed to find that the nearside rear brake drum was binding. With the hanbrake fully on, however, it would still not lock the wheel. Backed off the adjuster, but still binding so, with difficulty, removed the drum, . Found that the wheel cylinder had seized solid in a slightly expanded position - don't understand how this occured 
Even using a long bar as a lever in either piston groove I was unable to turn a piston, or fully retract either with G-Clamp. Hence the binding.Now to the main point of this posting. I checked the Microfiche, under 'Technical', for the correct rear cylinder and noted that the Rover part number for the very last ERA's was GWC 1131, but the earlier cars show an ERA part number, which we cannot cross reference. However, the stated Rover number was also given by a MiniSpares brake data sheet in an old parts list. In fact, it transpires that this cylinder was also used on one other Mini, the 1977/1978 Mini 850/998 with diagonal split brake system. The bore is given as 0.5625 in, which is much smaller than the usual cylinder size of 0.75 in, and will therefore require more pedal pressure to lock the back wheels.
I was still a bit sceptical that this was the correct cylinder, but ordered two equivalents from the local factor. On receipt, I immediately thought they were wrong, as they are fitted with large, 10mm, bleed nipple (Like the front calipers.) but the fitted rear cylinders had the small nipple, like all Minis I have ever worked on.
On reflection, I remembered that there was a works drawing of the ERA Rear Back Plate, showing modifications to the standard 'City' item. On checking, this drawing shows the original bleed nipple hole enlarged to 10.3mm and elongated downwards and the cylinder locating pin hole was also elongated downwards. To cut a long story short, when I removed the defective cylinder I found that the back plates had been modified as per the drawing and the new cylinder fitted perfectly
. New shoes completed the refurb and the brakes are now fine.For your information, the equivalent cylinders are made by at least two manufacturers, since the demise of Lockeed AP. and these are Brake Engineering, with the part number WC4415 (made in Italy) or Quinton Hazel under part number BWC 3099. I see that Moss and Mini Spares list them under the Rover part number, but without disclosing the make. (You pays your money and makes your choice!)
Since these cylinders are not normaly associated with a Mini I would advise that you all take a quick look behind the rear wheels, to see if a large bleed nipple is fitted. I doubt if many ERA's are still fitted with the original cylinders, so the odds are that they may have been fitted with incorrect items. Please check, as the brake balance will be upset by using the larger bore items. Please give feedback on this forum as to what you find fitted.
Best regards.
Pete K