All Spittys have discs up front and drums at the rear and the size of these two items stayed the same for all models 1962 to 1980. The differences between the models was with different front disc callipers with bigger disc pads and different slave cylinder sizes in the rear to balance the braking.
The other major change to the braking system occurred with the so-called twin system which was first introduced to America in the MKIII (1967), but in England about 10 years later with the 1500. The twin system is probably the most troublesome as more attention is needed to bleed and adjust the brakes correctly to give a good brake pedal.
All Spittys have 9" disc rotors up front and 7" drum brakes at the rear.
MKI and MKII Spittys have the smaller front disc callipers (Girling 12.P)
MKIII–1500 have the larger callipers (Girling 14.LF)
MKI, MKII and early MKIII Spittys have the larger ¾" rear wheel cylinders
Late MKIII and MKIV Spittys have the smallest ⅝" rear wheel cylinders
1500 Spittys have the in between size of 11/16" rear wheel cylinders
In England it was possible to purchase a power booster, but in Australia we didn't bother as we have stronger legs. Spitty brakes are not great, but if maintained well and with the use of high quality disc pads (Metal Kings and EBD Green) they will cause you no grief.