Morris Minor Owners Club

I have to change a rear wheel brake cylinder on my Series MM and compared the MM cylinder to a 1098 cylinder. The 1098 has a 3/4in. piston and the MM has a 7/8in piston. The 1098 is also considerably cheaper.
What would be the effect of fitting the 1098 cylinder?

Less braking force on the rear drums........ Doubt it will be significant - but you may have to press harder on the pedal - which in turn may make the front wheels lock on wet roads.......

I will try it out and if the back feels unbalanced I will check it on the MOT brake tester

That won't tell you anything - because the brakes will still work - just need higher pedal force. The MOT doesn't worry about front/rear balance.

if you only fit one side might that not cause it to pull to one side

That is what I intend to find out

Oh no! Don't fit to one side - I had assumed you would fit them as a pair - mad to do otherwise...........

When you only replace one side, with light braking you may not
notice much pull to the side. But hit the brakes hard and you
will wish you had replaced both sides.