An old fashioned Cooper S 7.5 inch single line system is fitted to my car. When I originally got the car running it had 3/4 inch rear drum brake cylinders. I quickly switched to 5/8 inch rear cylinders because the rear brakes locked up long before the front discs were really doing anything. The problem was still there, so I switched to 1/2 inch rear brake cylinders. That was acceptable, but still not great, with the rear brakes still taking too much of the load. The original Cooper S used the 5/8 inch diameter rear wheel cylinders. So my question is why won't 5/8 rear cylinders work for my car? Even my 1/2 inch diameter cylinders don't work very well. There is obviously something I don't know about how this system is supposed to work.
Sounds like you're missing a brake pressure regulator valve.
Quote from minispares....
All MK3 minis to 1978 except Cooper S MK3 which used 21A1774.
Fitted mainly to cars with 3/4" larger bore rear wheel cylinders.
The correct regulator valve for all MK1/2 mini plus all Cooper S inc MK3 is 21A1774 where most models had 5/8" bore rear wheel cylinders.
The only difference however is in the poundage of the spring,37H2009 which is weaker for this version to compensate for the change in wheel cylinder sizes.
See MS72 for adjustable type. The stronger spring in 21A1774 is available as 27H6620.
Supplied as 21A1774, but requires the spring changing (supplied
Either it's missing or rusted stuck wide open.
Mini Spares also sell an adjustable brake pressure regulator. It's pretty easy to adjust.....drive the car, slam on the brakes, if they lock up, run the adjuster in a little each time till they don't.