There 's nothing mechanical in the rear system that can "wind' up.. (never say never when talking Minis - and no mechanical adjustment at all at the front - so it has to be a hydraulic problem.

The best way to find out where, is to drive until the system locks up (and I'll bet that driving is not the problem, rather continued pressing of the brake pedal).  Then, crack a connection in the brake line.  I would suspect that you will find the line pressurised - and once the pressure is relieved then the wheels will spin freely.  I would (and did) open the outlet from the master - its the easiest to get to.  Beware.... the fluid will go everywhere - but there's not much.  So wrap the junction is a large lump of cloth to catch the drips....

I say "did" because I've been there. In my case it was NOS Girling master cylinder kit that was defective.  The hole that releases pressure when you come off the pedal had not actually been drilled....

In a single line system this affects all the brakes.  While you initially complained about rear problems, the later post indicates that the problem is affecting all 4 wheels.......   

Cheers, Ian

You have the correct piece of metal under the MCs so that is not the issue. 

And to add to the mix. I've found one PUP in UK with single line, and 7" discs and 3/4" rear brake cylinders, apparently braking fine!?

 

Some answers:   1.  no booster on this single line system.   

2. No Bulkhead blanking plate with 2 holes (ALA7530), so that may be the culprit? Do others agree?  

3. I have seen some mention of using .70" wheel cylinders instead of .625"; what is everyone using if they have a single line system?  And I'm pretty sure I've go 3/4" on this PUP presently, which is apparently too large, but does that have an effect on the front discs (see next question as followup to this question)?  

4. I jacked up the PUP and on stands all 4s.  I tried pumping the brake pedal, it has a desirable length of play before getting hard pressure.  I couldn't get the rears to lockup, maybe just a little stiffer to turn. But the fronts which were still pretty hard to turn (even after days of sitting) became even more difficult to turn after pumping the pedal numerous times.

4. And this is the correct photo of the rear subframe and tubes, presently on the vehicle: 

 

You made no mention of a brake booster, but if you have one I would look there first. I have had this problem before and that was what I found to be the problem.

Nice looking job you are doing on it.

Opps good catch.  That is an old photo .. I don't remember why but I did run the line on left initially ... then changed to right side (now on right) which is best for this RHD, and allowed fuel line to run on left side which worked best too for the dual carbs.

Not your issue.....but I think your brake line is supposed to be on the right side under the length the truck.....maybe different for pup or rhd(?  )

I still can't get back to the car to evaluate for some of the suggestions that have been offered.  But to answer some of the questions:

I don't know yet if both front and back are locking up.   That should be evaluated obviously, which I did not.

All parts are new, all lines, all braided hoses, new Master Cylinder.  I bought a brake master cylinder.  retty sure I installed it correctly, I've done so on two other Minis that needed new MCs too; but I'm sure going to check to make sure it isn't a clutch MC.  

The change to 5/8" wheel cylinders, was mentioned in other threads, and needs definite consideration; I'm pretty sure I have 3/4" on presently, but need to double check that again. 

I couldn't verify for sure what proportioning valve I have.  The numbers on the unit (looking at it under the car with a mirror, and trying to read the numbers in the reverse on the mirror) didn't match up with the ones typically for sale now.  The one I listed initially, is the one I think I have. 

I know I installed a Master Cylinders Plate Gasket, And Firewall Upper Stablizer Bracket was installed.  I'm not sure, and may not have installed the "Bulkhead blanking plate w/ two holes" as was suggested by Dr Mini.  I'll definitely need to check that!  For parts like that, I cleaned up the parts that came with the mostly "2/3rds rust" vehicle.  I'm asking myself, is it possible I didn't have that part?

And to add to the mix, here is the plumbing at the rear subframe (see later post for photo).  I'm not sure if I should re-do the plumbing.  I used the pre-made lines that came when I bought the Pup.  So the length was not custom fit, thus the bend and curves to shorten where needed.

 

I'll give some feedback when I get back to the car.  Thanks again for the responses, it is helping sort out the issue, I'm sure.

I have seen this happen too. If you can't remember how old they are, I'd replace them.

Just to add another item not yet mentioned. How old are the flexi hoses and are they in good condition? I have worked on a few cars where these have turned to mush on the inside due to age and caused the brakes to lock on.

Leaving the MC/pushrod issue aside...as stated above...you must use the narrower bore rear wheel cylinders when S disc brakes are installed (not more than 5/8" / 0.625") and standard proportioning valve.

Thanks for replies, it will be a few days before I can check theses ideas out. All have validity relative to my circumstances it seem.

What all did you replace when you did the brake job?  Did you replace the MC?  Are you sure you have a brake MC and not a clutch MC installed?  If you are sure, there is a metal plate that sits under the MCs.  I have seen folks leave this out when they build a car and it is very much needed to make the brakes work.  Without this metal plate, the brakes will take a few hits then lock up.  If you let it set for a little while it will eventually unlock, but the next time you drive it will lock up again.  Cupcake indicated it could be too long of a pushrod in the MC, well, leaving out that plate does the same thing as having too long of a pushrod.

5/8 rear cylinders are what is needed.

Are you sure the front brakes are not locking up as well? If both front and back are locking up the likely cause is the wrong (too long) push rod in the brake master cylinder ie a push rod from the taller Cooper S master cylinder in a standard master cylinder. If its just the back brakes that are locking up i would suspect the brake bias/proportioning valve is the problem. You can't just remove it (question number 2) because without the proportioning valve the rear brakes will lock up on hard braking with disasterous results. I did 180 degree spins twice at the same traffic light  because of a defective brake bias valve.

Have been having problems with rebuilt rear brakes, (with new 7 1/2inch discs front), Classic 72+ Pickup install process.      Before the drive I checked the rear wheels and found they turned freely, as they should.  Went out for a test drive, around the block, less than 1/4 mile. (It was running pretty good, still needs a little tuning).  Had to make intersection turn at about 100 yards, touched the brakes a couple times at that point, and the engine started laboring.  It got worse, but was able to get back into the garage.  Engine could barely move the vehicle.

      Immediately after parking, I jacked it up, and found that the rear wheels were locked up tight!  Couldn't be turned on either side. They became somewhat looser when rear wheels were checked about an hour later, but still pretty tight (just not totally locked tight at that time), and turned freely the next day.   I did not apply the handbrake during this drive. 

      I have a single line system with a GMC0171 master cylinder, 3/4" rear brake cylinders, and I think: Brake Proportioning Valve, Mk1-2 and Cooper S Mk3 (21A1774).   No regulator valve.     I have reviewed many of the brake related forums, and just don't see the answer in my assembly.  

Here are the questions:

      1. Is it possible the the proportioning valve is preventing fluid from returning back toward the front (and the master cylinder)?  

      2. Can I successfully, and safely remove the proportioning valve entirely from my system, either with a T connector in place of the rear subframe mounted proportioning valve, or by removing the innards of the proportioning valve.  

      3.  On our host website and article states: a) You can only use the MS72 adjustable brake regulator/bias valve with a front/rear split system.  While b) the Proportioning Valve MS72 sales information indicates that it is the answer to preventing locking up the rear brakes before fronts when moving during a braking operation.  Again, my present install is a single line system, so can I and should I use a MS72 or similar?

       4.  Any likelihood that I have air in the lines even though the pedal isn't spongy, and that could cause a problem?