I decided to drill the post hole, and the initial testing suggests I got the alignment of the brake cylinder suitable.

Bleeding complete, initial test on stands, brake system appears to be working the way it should.  Replaced master cylinder, replaced proportioning valve, replaced 3/4" brake cylinders with 5/8" brake cylinders.  Next test< hopefully tomorrow< is a road test.  Again, the culprit seemed to be the proportioning valve, based on test of pressure at points in the line before the proportioning valve and after the proportioning valve.  Fluid flow was greater after the valve than it was before the valve, when the rear brakes didn't seem to unlock.  Now with adjusters fully loose, and the new parts, the rear brakes get loose enough to turn easily.  

The S rear cylinders have the dowel pin in a different position to avoid errors on the assembly line.
I used to redrill the backplate, but lately I use the mill to add an extra dowel hole to the cylinder instead.

As I started to work on the LH side, I could read a number on the plate, somewhat more clear than on the RH side: 113516 LH, which brought up using browser search a product: 21A1060, which indicated "This now replaces all earlier types and therefore some will require extra drilling to allow for the wheel cylinder locating peg which was on the other side."  So mystery solved, and my decision to drill is the correct decision, I'm still hoping I get it to align acceptably.   

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As I'm re-doing with new Master Cylinder, new Proportioning Valve, and new rear brake cylinders 5/8" I discovered the 5/8" rear brake cylinders will not fit as the 3/4" rear brake cylinders did.  The hole for the peg on the brake cylinder is on the wrong side on my back plates.  Why?  When I initially installed 3/4" brake cylinders .....how did I get new 3/4" brake cylinders that fit, and the 5/8" won't without drilling a hole for the little peg on the backplate?

This first picture is the plate as it had been installed with the 3/4".  The picture includes the new 5/8" to show how the peg is on the left side, while the 3/4" that is being replaced has peg on the right side in the photo.

I decided to try get the project moving ahead so am drilling a hole for the peg, hoping that the alignment for the the cylinder will be ok!  See in this next photo the hole drilled and one of the 5/8" cylinders fitted to demonstrate the position.  I have completed, thus far, the install of this R rear backplate, and so far it appears successful.    I'm anxious to get fluid back into the system to see what happens, but while taking a break I thought I'd post this additonal part of my brakes saga!

I don't think the orientation of the valve would matter as long as the lines are connected correctly. As the valve is expensive i would try to by pass it with a three way connector then re bleed the brakes and test, if they still lock as long as the hoses are new or good the master is probably bad. Again don't drive it with the valve by passed.

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Malsal, I loosened the master brake line connection, with this result: a little flow for short seconds of brake fluid; and rear brakes freed.  Then pumped brakes and rear drums tight again.  

 Then  released line at the proportioning value from master brake cylinder (essentially the same line), with the same result, brakes freed. Then pumpd brakes and rear drums tight again.

  The released line at the proportioning valuve to one of the rear brakes, with more flow for longer seconds (of brake fluid).  The  brakes were freed.  This step is what my very knoweldgeable mechanic friend at church today, suggested, as he was suspicioning the proportioning valve.     

 So, if I'm diagnosing anything correctly, it looks like the proportioning valve is problematic, but this is contradictory to several other sources including on this tread.     But I got to wondering, is it possible to install this 21A1774  proportioning valve incorrectly, such as upside down?   

This side presently installed upT

his side presently installed down

"..... I released pressure to the line, and found I then could slide the drums (both new and old/used) on without difficulty and they turned fine.  Then again pumped the pedal a few times, and again the drums is tight on the shoes....."

In that case I think we can rule out oversized shoes.  (You can get shoes with thicker lining to compensate for metal skimmed off the inside of the drum).  As has been pointed out.. wider (ie front) shoes don't work at the back as they are wider than the space between the mounting slot and ridge on the backplate.  They will look like they fit but will be tipped sideways so only a small edge will bear against the drum... 

So that only leaves residual line pressure...  In my view (others may see other options) there's only the master cylinder or the pressue valve (on the rear subframe). The pressure valve only has a spring , a piston and a seal - hard to see how that could go wrong....

The master problem could be an excessively long actuating rod or a defective return valve.  I know its new - but so was mine..  I think you will just have to remove the master and investigate...

Cheers, Ian

I meant to add, if the color of the brake line fluid isn't obvious, I'm using DOT 5 silicone brake fluid in this system. I assume it should perform with these components the same as would the more caustic Dot 4 fluid.  I've got an expensive paint job on the vehicle, and chose to use the silicone fluid.  Any clues this is part of the problem?  

The drums are steel. I've tried to measure the internal diameter with tape measures, the best I can do: 17.7 cm or approx 6 15/16 inch.  I have no idea where to find what the diameter should be.   I keep thinking it is either these new drums that are an issue, or it is the shoes that are too thick.  The widht again is standard for rears,  1 1/4", I have checked that measurement.    The shoes are 0.5 cm, or 3/16 inch (the material only).   (I believe typically front drum brakes are 1 1/2", used them on my dual leading front drums on the Moke. 

Got to be a hydraulic problem, either the Master, bias valve or the flex hoses. As suggested i would loosen the master brake line connection first and see if that frees the rears up.

The drums are difficult to push on, pull off.  When the pressure is on they are too tight to spin correctly, and tight to pull off.  There seems to be a size issue, as well as maybe a Master Cylinder issue.  I have received repeated indications that the proportioning value (bias valve) is unlikely to cause the fluid to not be released, and a broken proportioning valve is unlikey to cause lack of release of fluid pressure.  All four flex hoses are new.  Master cylinder is new.  I do have an old Master Cylinder, but not in great shape.  Thinking about trying a new one.  I've swapped the caps between the clutch and brake masters, without any difference in results.  

The adjuster wedges are in correct. The reason you can't see is because they are not adjusting anything, they are totally loose!  That is part of my issue, especially on the R rear which addresses your second question about "oversize rear shoes".  They seem to be oversized, but I'm not aware you can get anything with thicker material?  They are 1 1/4", and that is all I really understand about them.   I took off all adjuster pressure to the shoes.  I released pressure to the line, and found I then could slide the drums (both new and old/used) on without difficulty and they turned fine.  Then again pumped the pedal a few times, and again the drums is tight on the shoes.  

If those are alloy rear drums, the internal dimensions may be different than those in Larry's report. The tolerances might be tighter and less forgiving.

I can report that the wider (1 1/8"?) shoes  for TLS front brakes can be crammed in rear drums (1"?) and the rear brakes will work albeit not very well because the shoes are canted. My current Cooper S was so fitted when I bought it.

Assuming that all the four flex hoses are new or confirmed good the master cylinder could be suspect along with incorrectly sized drums or the bias valve malfunctioning. Do you have the old Master to fit for testing purposes, measure the drums and i don't know how to test a bias valve apart from eliminating it for testing purposes (not driving of course).

Ian: by "oversized" do you mean larger diameter or wider? I could see where a shoe too wide for the drum could be rubbing on the inner face of the drum, causing it to wedge between the drum and the back-plate when it warmed up.

If I understand the last report, the drums were braked even with the adjusters pulled back. Were the the drums were difficult to remove? If they were difficult to turn but easy to pull off, then it would appear to be a sideways clamping action (the inverse of disk brake action). This would suggest that the shoes are too wide or the drums too narrow inside. 

A hypothetical thought: The overnight 'delayed action' binding seems to point to a hydraulic flow issue - the master cylinder not being relieved and the pressure being held in the line, which due to a slow or malfunctioing regulator or balancing valve woudl let the pressure slow-leak to the rear cylinders, applying brake pressure. ( I did say hypothetical!)

I can't really see..... but are the adjuster wedges in the right way around????

The only other thought that occurred to me is that, you may have combined new drums with oversized rear shoes??

Cheers, Ian

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I wondered about the 7" Discs example, DRMINI, they are weaker, so that makes sense.  

I haven't eliminated the pressure issue, per 1963SV2's comments.  And to enhance that thought process, I found tonight when pulling the rear brake drums, that the resistance to turning the wheels / drums was greater tonight than it was last evening, after sitting for 24 hours!  Again, last night, I had pumped the brake pedal numerous times trying to get the pressure up so I could duplicate locking up the rears, which was relatively unsuccessful.  So, it didn't happen right away, but later!    More on this further below in this reply ......

Thanks DRMINI, I too confirmed that the single metal plate under the master cylinders is correct.  I initially understood that I needed to add a second (blanking strip with holes) which I learned is incorrect, and would not fit the master cylinders.

Now for an additional discovery, actually a re-discovery.  Upon removing the rear brake drums, I found (again) that the drums are extremely tight, even when the adjusters are completely positioned with no presure on the brake shoes, and the brake cylinders appear to be suitably depressed too.  I tried a couple used drums and found them to be somewhat more cooperative, but still more rubbing resistance than would be what I consider normal.     

R Rear  R Rear  R Rear  L Rear

Glad it is Saturday, time to try some more of the suggestions.  Thanks

Well, 7.0" discs are Cooper, not Cooper S. They work with 3/4" rears OK as they brake poorly.

Cooper S (7-1/2") front discs DO need 5/8" rear cylinders. As I found out, on a freeway @ 110KMH in the wet with 3/4" cylinders on...!