5/8" (as per minimans) for 7.5 discs on Cooper S with the right proportioning valve and a servo.

9/16" (as per CooperTune) will also work and I've gone as small as 1/2". Both I've used w/o a servo.

5/8 rear cylinder for Disc brake cars as far as I remember..................

So, what size should my rear wheel cylinders be for my Morris Cooper S 1968?  I have 3/4 inch now.  The old ones were .82".
Dogscarf.

3/4 wheel cyls are wrong and will cause an imbalance in braking front to rear. I'd suggest the 9/16 cyls. Steve (CTR)

Fellow Minioids........  I am still working on my rear brakes.  I removed the two rubber hoses.  I tried to pass a bicycle spoke through to no avail.  Clogged.  I opened the hard lines at the brake balancer on the subframe and got fluid.  So, that appears to be ok.  My two wheel cylinders were frozen with no fluid in them.  I have new ones and I installed them, they are 3/4 inch.  Is this correct for a Morris Cooper S in 1968?  If not I will get the correct size.  I broke a hard line to the wheel cylinder so I will either make one or buy one.  ANyway, things are looking up and I should be able to get my brakes pumped up next week when parts arrive.

Dogscarf

1968 Morris Cooper S

Of the three balancers I've taken apart, two were roached inside.  There is a spring and piston valve with integrated seals.  The seals were rock hard and in pieces.  The kits to rehab the proportioning valve are out there but uncommon. 
So, I hope it's bleeder nuts that need to be drilled on yours, as Lankford describes!    

Good outlook = Thanks for the Followup

I had this happen once on one new rear cylinder about 12 years ago. No flow.

I unscrewed the bleeder all the way, still nothing. Looked down inside and saw that the hole had not been drilled or tapped deep enough to enter the wheel cylinder itself. It was just a small amount of metal not carved out, maybe just 1/16th.  I just punched through it it with a small drill bit (no drilling needed) and voila, out squirted the brake fluid.

Quality control issue at the manufacturer. So much for inspection, theirs , or mine. Never trust any new part until well-examined.

I have run into this several times recently. I get the fronts bled out and there is no pressure at the rear. I have two brake pumpers. One I can't seem to get to press pedal slowly up and down. The last one was a drum to 7.5 disc convert with 3/4 rear cyls. With 3/4 rears rather than the smaller correct cyls the rears tend to lock up quickly. I ended up backing off the shoe adjustment half a dozen clicks each side. Got a my slow pusher of pedal and the rears started to show pressure when bleeding and the three inch long air bubble ran right down tube into catch jar. Maybe it was just time but after working with it several times over a couple of days I was ready to try the fitting by fitting approach. Steve (CTR)

It is probably the valve as you suggested.  There are several ways to work around this.  I suggest the following.

Locate the valve on the subframe and crack open the fitting where the line from the front of the car enters the valve.  Push the brake pedal down slowly until fluid comes out of the joint then tighten the fitting back.  Repeat this for the discharge lines exiting the valve.  Move on to the flex lines with the same process.  Once you have done that, resume normal brake bleeding.  I suggest you do not use an EZ-bleed or similar that pressurizes the system.  Use the two person flush method where you open the bleed nipple and call to an assistant who SLOWLY presses the pedal down.  When the pedal is down they call "down" and you close the nipple.  Once it is closed you tell them to release the pedal and when it is up, you repeat the process until no bubbles come out in the drain tubing.

When bleeding the rears i have found the higher the back end the better they bleed with awkward ones. Bleed from the front to the rear one connection at a time you can usually find the air lock. Also have you replaced the rear hoses as they collapse from the inside when old.
I use a small flat screwdriver to install the circlips.

Fellow Minioids,  OK, I now have a hard brake pedal.   No leaking servo  anymore as I removed it.  No leaks anywhere.  New pad and wheel cylinders on the rear.  The front brakes work but the back do not.  I open the wheel cylinder bleeders and no fluid comes out under pressure.  l   wonder if it is the brake balancer attached to the subframe????  Maybe a piece of grit or dirt is jammed in there.  Thoughts???



By the way, I found an easy way to attach the rear wheel cylinder clips with a set of channel locks.  The special tool that tightens up a wedge against the snap ring to spread it is a giant waste of time.  Into the trash bin. 
Dogscarf

68 Morris Cooper S