To further clarify. All the four speed and the automatics thru the change in about 1980 or 1981 use the larger bore four speed brake cylinders. What happened was the automatics used the four speed axle assembly. At the end, to use the same rear end assembly, Triumph switched all models to the five speed rear end and changed the internal ratios to suit the transmission being used. Where the five speed had been 3.90:1, it was altered to 3.45:1 with an additional change to the fifth speed gear set in the gearbox. The automatic rear end ratio was changed from the old 3.27:1 to 3.08:1, the same as on the TR8.
So, in your case, the four speed and automatics brake cylinder is the same when using the early series rear axle assembly. Please refer to your reference materials such as the V-B catalog, The Roadster Factory parts catalog, the Rimmer Bros catalog, Rockauto.com online catalog, and any other source you can find in order to ascertain the correct part needed for a given repair. Otherwise you will wind up with a useless parts inventory.
Mildred Hargis