I cannot answer concerning the split master cylinder.

I suggest that since the system has been open that you crack each joint open from the front to rear of the car.  Start with the connection of the pipe to the MC.  Crack it open and pump the pedal gently until brake fluid drips out.  Close that joint and move to the next one down the line.  Keep doing that, going from joint to joint until you reach the rear wheel cylinders.  

I mentioned I don't use the Pump-pump-pump-crack method of bleeding brakes because it does tend to shift the proportioning valve and close off flow to the rear cylinders.  Any fast, firm pumping will do that.

I'm not sure if you meant a long time between working on the Mini or to get the cotter pin spread but yeah... life tends to get in the way.
The master cylinder seems to be primed and holding pressure but I can't get any fluid to drain from the rear bleeders. Before removing he old master, I'd left the right rear not snugged up and it drained a bit onto the floor. The left rear seems bone-dry - I had removed the bleeder completely to clean it and make sure it was not plugged. I'm beginning to suspect that the proportioning valve is stuck. I'm also working alone as my wife is out of town for a funeral, so I've no-one to work the brake pedal at the moment.
I will probably be working my way through the system to flush and replace fluid and bleed air out of each section.

Question: On a yellow-band master cylinder, does the upper section serve the rear or front brakes? (System is split front/rear with 8.4" disks on the front.)

Boy that took a looong time!! I'm glad you got it done and your backs in good shape

The new master cylinder is IN! It only took me 15 or 20 minutes to get the pivot pin in and then maybe another 45 to an hour to get the split (AKA cotter) pin in and spread... that little bu**er is slippery.

I used a short scrap piece of 1/4" steel brake line tubing to align the holes from the clutch pedal side, then slipped the pivot pin in from the correct side. Pretty tricky task as I could only get one eye on it so had to guess at distance to line the pin up with the tubing. I was surprised when it actually slipped in. I was doubly surprised to see the hole for the cotter pin actually lined up properly. It was easy to slip the cotter pin in and get it turned around but a challenge to spread its wings and bend them back.

Tomorrow I start the bleeding process. 

I'm confident the fronts are 3/8-24 but I don't know the length.  The rears?  Stock ones are 1/4-28 (UNF).  However, a Mini Spares link says some AFTERMARKET modern wheel cylinders have 7mm threads.  Best to measure the thread OD (and length) to be sure. 

While available online, many auto parts stores carry Speed Bleeders.  Measure what you have and compare to what you find in the local stores. 

I use stock bleed nipples and have bleed hoses with inline check valves instead.  The check valves were free while I would have to buy Speed Bleeders so it was a simple decision for me.  If I had to pay for the check valves... I'd go with the Speed Bleeders.

Doug: I had a look at the "Speed Bleeder" on-line - very interesting. They apparently come in a myriad of sizes.
Do you know which sizes would fit  mid 1980's 8.4" disk calipers and rear drum cylinders?

Yes, I did think about that. I came to the conclusion that the bracket bolts have been in since the car was built and may be rusted in. I didn't want to get side-tracked on non-brake problems. As it is, the left side demister hose and funnel/nozzle came adrift, so once the brakes are done, the dash card has to come out.  The plywood idea ended up being quite comfortable and very stable, and the door weatherstrip is also protected.

Remove the seats with the brackets attached. Only a couple more bolts, but it solves the problem of the brackets in your back.

Progress report: The old master cylinder is OUT (with no apparent injuries!).

Heeding the advice about a pillow etc, I had a think about it - I concluded I could hold my head up but lying over the sill and cross-member, not to mention the seat brackets, I concluded a flat surface to lie on would be beneficial. Conveniently, there was a scrap piece of plywood standing right next to the Mini that fit from the gear shift to about a foot beyond the door sill. With some wood blocking in the rear footwell and a bench and blocking outside the car, which is on jack stands, I had a platform at convenient sitting height where I could sit down outside the car, lie back and get a great view of all the wires between me and the top of the brake pedal. Once I figured out where I needed to see and repositioned about 8 or 9 times, I could actually see the cotter pin. I was able to get it out using mostly patience, a piece of what I think is 1/8" welding rod (I don't weld) and some standard length needle-nose pliers.
The pipe fittings to the master cylinder came undone with WD40 and gentle persuasion, as did the hold-down nuts. Once it was out to the bench, I drained the old master cylinder - pretty murky!

Without reading all the posts.....  

Did someone recommend bench bleeding your new master??  Well worth it.

I think DOT 5 (synthetic) fluid was pretty rare back in the 80s ... so DOT 3/4 would be pretty safe bet.  However, what ever it is, its going to be petty manky by now and (no doubt) needs replacing..  

It would be madness not to go for complete replacement... Which is where the "Ezibleed" comes into its own.  Especially if you choose a fluid that is a different colour.  You can use any of the old methods ... (at least those recommending doing one wheel at a time) just keep pumping until the new stuff comes though.

Basically the numbers indicate the fluids boiling point - the higher the better.  As long you stay way from DOT5 unless you're going for complete flush and replace ... and even then I would be using new seals ALL 'round - DOT 5 is different chemistry to 3/4/5.1 and is not compatible.  DOT5.1 is compatible with 3 and 4 and has he highest BP... but is not that common and (I seem to recall) more expensive. 

And... just a personal perspective .. but I find the old method of sticking a length of clear tube into, say, half a glass jar of new fluid and gently pumping away has a lot to recommend it... You need two people (or maybe a remote camera) to watch the fluid coming out while pumping the pedal.  On the downstroke you'll see old fluid and bubbles while the upstroke will suck a bit of the fluid in the jar back up the tube (the bubbles float away).    Just keep pumping away gently until you see only clear fluid being expelled...  But, MAKE SURE you check the level in the master cylinder regularly.

I do have an Ezibleed but you need to be VERY sure that you get a good seal between the master cap and the device ...and don't use too much pressure (don't ask me how I know .  The vacuum method also works but I seem to have lot of trouble sealing the open bleed valve so that air doesn't get sucked around the bleeder valve threads...

Cheers, Ian

Update: tomorrow I get started, probably after my physiotherapy appointment. I have received the pretty, new master cylinder and have re-read all the guidance so far.

One question: Assuming the fluid has never been changed, in a mid-1980's Mini, would it have DOT3 or DOT4 and since I will be flushing the system, wold it matter?

Second question: Is DOT3 any more or less hydroscopic than DOT4?

the long 15 inch or so hemostats make it easy from underneath. The right tool for the right job. It fits right in there.

Make sure the flex hoses have not collapsed internally.
One time with a rusted stubborn split pin that would not come loose i was able to remove it from the top side with the master loose you can just about reach it with the brake pedal firmly pushed to the floor and a long set of needle nose pliers or hemostats.

Thanks for the reminder on the pillows! Not looking forward to those gymnasties - my chemo a couple of years back left me with "fragile" muscles - easily strained. It takes me about 3 days to recover from what used to be moderate exertion.

Update: rear bleeders cracked free without incident. No fluid came out of the left one even when completely removed for cleaning. I thought it was plugged but it wasn't. I suspect the master cylinder's seals have come adrift from the piston/shuttle, preventing even leak-down by gravity. Even a tapety-tap-tap with a MFH ( not the BFH) on the sides of the MC didn't free anything.

Swindrum. I use a floor jack to raise the car in the back and crawl under and find the bleeders. Same thing in the front. No taking off the wheels. Dan As for rebuilding the master cylinder I've done that before and it is doable with not a lot of problems, it can be done. EDIT And yes you should use some jack stands so the car doesn't Crush you and use about a 15in long set of forceps to get the cotter pin in the master cylinder that's under or above the pedals. And have fun working on your back under there LOL. And I got the hemostats at a place called " tool pawn". It's a decent-size place and all they do is tools.

Another bot for the EZ-Bleed. Not sure how I lived almost 50 years without one. There is a reservoir that you fill with fluid, then just run around the car opening bleeders. Takes 15 minutes, 10 of which is jacking up the car, finding all 4 jack stands, and taking the wheels off...

if your master cyl is done, I highly recommend replacing it, they are under $200 and are a colossal pain in the butt to rebuild...
i use the yellow band dual masters in everything, from drum braked 998’s to our twin-cam race Wolseley.

Thanks for the continuing advice, Doug.

Recently in another thread, someone advised that if one was going to the trouble of taking out and putting back a master cylinder, it might be better to be putting in a new one in case the rebuild was less than satisfactory. If I don't recover any pressure, I'll very likely get a new master cylinder. 
Between steady rain today and sore muscles that rendered me useless (okay, maybe more useless than usual), nothing happened with the Mini. Maybe tomorrow.

If the pedal has little to no resistance then I doubt you will be able to bleed the system.  However, it won't cost much to try.

As for replacing the MC, do they not sell rebuild kits for it?  If you remove the MC and tear it down for inspection you'll know soon enough if it appears to be rebuildable.

Doug: I was afraid somebody would say that. Is it worth trying to bleed the system to see if it comes back? Meanwhile I'll start shopping for a master cylinder.