I would never personlly use pliers or a pipe wrench on a piston. I open the vice on the toolbench and place a chunck of plywood against the back end with the caliper resting against the front. I then gently blow air into the caliper (with the caliper broken into 2 halves). I cant say enough how GENTLE you must be, if you push too much air too fast you can loose a finger or worse, damage the piston when it hits the floor. Another "top tip" as the Wheeler Dealers say... when completing the rebuild, the metal ring that holds the outer rubber seal in place are a pain to fit. I used a C clamp & piece of wood. while applying steady even pressure one corner always refuses to go down. if you arent careful you will crease the metal & be screwed. I take a small chisel and tap-tap-tap from one side to the other, add a bit more pressure from C clamp & repeat w/ the taps and prressure until it is completely seated. Cheers Rob

I know this is an old post, but I just wanted to highlight a bit of great advice that I found in searching through back posts. I had a soft pedal, bled the system twice to no avail. Adjusted the shoes according to HBOL..."turn the adjuster till it locks up, then back off a quarter turn". Still pedal to the floor. Slept on it, then did a search and found the above advice. Tried it this morning and it's all sorted. Nice hard pedal... feels like new shoes again. I'm amazed at how fractional the difference in adjustment is between hard pedal and none at all. Searching around, it seems like this is a pretty common situation, leading to lots and lots of unnecessary drum bleeding. 

Nice advice. Saved me a lot of headaches. Thanks, Doug.

did you ever find an answer to your question????? My only remaining question is should the brakes be bled with drums tightened up or drums loosened off.  I have been told that sharp pumps and holding the pedal pushes air bubbles out to the end of the lines....

i am looking for this answer also, i have a 4 wheel drum brake mini, and i can't get my brakes bled, can someone share there secrets with me, i have checked OLD posts and i can't find an answer to this question, what is the best way, i have all new wheel cylinders, metal lines, braided flex lines and a new master cylinder, everything is new and i still can't get a good pedal, i have checked my brake shoes and they are all installed the correct way, all new springs and hardware on all 4 corners.  

Before you cut the bolt, put a nut on the part of the threads you will be keeping. Cut off the threads and dress the last thread with a file so that the end looks like a regular bolt. When you're done take off the nut and it will straighten out any damage that has been done to the threads. Running a die on the threads works but it can be very difficult if the first thread is mangled.

Use the blue Loctite and make sure you torque the bolts to whatever is recommended. Red Loctite means you will never take the caliper apart again.

Kelley

Thanks for that.

Based on the threads per inch and the head hex size, it sounds like they are probably high-strength 3/8-24 hex head bolts.  Given that, you could probably buy ones that are NOT threaded full length making sure the unthreaded portion is the correct length or longer.  Cut off the extra threaded length, then chase the threads that are there and  (if necessary) cut a few additional threads in the unthreaded portion to get all the lengths correct.  Since these would likely be Grade-5 or Grade-8, you'd need a new, sharp die and good cutting lube to get an acceptable finish on any threads you have to cut.

Let us know how they turn out.

Here's the quote from what I was told:

They are a special bolt in that the unthreaded part must be the correct length.

I've measured them. I think the thread is 24 threads per inch. The unthreaded part is 1/4" or 3/8". The overall length of the bolt is about 1-5/8", and the head is 9/16".

That's all I know.

When I re-use them, I'll probably use some threadlocker. Loc-Tite blue if I'm not brave, loc-tite red if I'm feeling confident.

Ken

You got me curious.  What did you learn about the threads?  Are the bolts Whitworth or something similar?

Just an update:

Turns out the bolts that hold your calipers together are as rare as hen's teeth. They are a weird thread, plus a weird length. So - don't throw out old ones.

I've also talked to some people who rebuild these for a living. As long as they pass visual inspection, re-use them!

I'm going to re-use mine.

Waiting for parts.

Ken

Doug -

I think I have that disease. I can see two MKI servo units on a shelf - both have leaks, along with another engine and a bunch of other miscellaneous bits.

I think I'm going to try the rebuild after all. I was able to get a small pipe wrench on the lip of the piston and rotate it a bit, and once it got a little loose was able to pull it out with some pliers.

Both are out, and I've cleaned up the seal grooves with the wire brush on my dremel tool. I've also cleaned up gunk on the wall of the cylinder. Looks pretty good.

I'll look at the bolts and report back on my findings.

Thanks for all of the help! I appreciate it.

Ken

Ok, buy the new calipers if you feel that's best for you.  However, if you weren't already aware of the weird behavior that develops when you own old British cars (and old cars in general)... don't throw the old calipers away.  In all likelihood they can be rebuilt by someone else if you don't want to do this.  There is certainly a market for the used parts, they are not trash.

New pistons (particularly if you go for the stainless steel ones) are great.

As for the bolts, this may seem heresy to some but if new bolts are not available from our host, they are likely to be available from regular nut and bolt houses.  I have not taken calipers apart but I suspect that they only used standard size/pitch high-strength fasteners.  Look for "tick marks" on the head of the bolt and count them.  Add "2" to the number you count and that defines the SAE grade of the fastener.  (Grade-8 = 6 tick marks, Grade-5 = 3 tick marks... both are common).

There are many ways to get stuck pistons out.  Since you have already taken them apart, put them back together.  Put the first piston back in the bore it came out of and hold it in the caliper with a large C-clamp.  Find a larger grease (Zerk) fitting and screw it into the caliper where the brake hose would normally go.  Connect your grease gun and pump away until the stuck piston comes out.  Obviously very thorough cleaning to remove the grease follows that step.

If you don't want to go to that trouble, consider that you are going to replace the piston AND the caliper is already in two parts.  Weld a bolt to the inside of the stuck piston and use a slide hammer to pull/pop it out.

There are other methods, but these are the two I would recommend based on what you've already said.

Doug -

Yep - I did the forbidden - I split the calipers. Not only do you need than internal seal (which I found at a Mini parts place), but you are also supposed to replace the bolts, since they stretch.

I also sourced new pistons, if that was the way I was going to go. There is also a seal kit just for redoing calipers.

But - here's what is pushing me to "new" calipers -

1) I know that pitting in the cylinder is probably fine, but the bolts to put the caliper back together are NLA.

2) I can't get the piston out of the "other" side.

3) I'm lazy, and new calipers are easier.

No worries about timing. This is a toy. If it sits for a month while I save up for some calipers and rotors, so what? I'm doing a headliner right now anyway, so it's not like I don't have things to work on.

Ken

Ken, you split the calipers?  The rebuild kits typically don't contain a new seal to replace the one you should have found between the halves.  I have been told they can be ordered but I don't know a part number for them.  Hopefully someone reading this will post the part number so you can order it when you order the rebuild kit.  If you cannot find this new seal, you can probably find a suitable sized O-Ring made of EPDM (do NOT use a standard Buna-N o-ring).  EPDM o-rings can be ordered from most bearing supply houses and from McMaster-Carr online.

If the caliper pistons are pitted, fit new ones, you cannot clean up old ones.  The bores in the calipers can be pitted a bit and be OK.  The sealing area that is important inside the calipers is/are the surfaces where the square section o-rings seat.  Make sure you remove any corrosion from those surfaces before fitting the new seals.

Fitting the dust excluder retainers to the calipers is the worst part of putting calipers back together.  It's bad enough that you may wish to take a piston to the hardware store... checking it against the ID of any pipe fittings and such you can find until you locate something the piston will just fit inside.  This will make more sense when you are ready to put the parts back together.

Dang that sucks...maybe someone on here has a used one that is still in good working order.  Might save you a few bucks and get it up and going faster.

Well - did some more work, and I think I have it figured out.

I pulled off one of the calipers. I really struggled getting the pistons out of the calipers. I finally split the caliper, and I got one side off. I think I know what's wrong. There was a ton of corrosion inside the caliper, as well as a bunch of dirty brake fluid. Even though I was bleeding clean fluid, I don't think it was circulating through the whole caliper, and there may have even been air in there some how.

So - I put the car away for a bit, and I'll just have to save up and order new calipers at some point.

I guess for safety sake, it's not bad to re-build the entire brake system.

So - We'll start pinchin' pennies.

Ken

For those that aren't sure what type of rear cylinders they have, simply post a pic online and someone should be able to ID them for you based on where the pin on the backside (the one that keeps the cylinder from rotating).  If I remember right one has the pin on the left side, another has it on the right side, and the last has one in the middle.

 

But if you're new ones fit the holes in your backplate you have the right one.

Dan -

The brakes seemed to work fine back in Feb when I started taking things apart. Since then, I've done stainless braided lines on all of the corners, and new copper lines across most of the car.

My car (72 Inno) has the Lockheed "Cooper S" master, and that's what I've replaced it with.

The old cylinders in the rear had a bore inside diameter of 0.622". That was so close to 5/8" that I ordered the 5/8" wheel cylinders.

Looking at the front calipers, they look pretty rough. I think re-building them is a good idea no matter what.

Graeme - nice! You are lucky!

Ken

I checked all my brake setups with the pictures and found the RH Rear was not installed correctly.  I fixed that.  I found that the front drums on my car were difficult to get right and impacted the brake pedal travel the most.  The rear drums were relatively easy to set up with each tire rotating about 1 revolution when set up.  Everything on mine is new so I adjusted the fronts to where I can rotate the tire with some effort and took my car around the block.  First drive in 2+ years!!!!  Brakes worked - some pedal travel but OK for stopping.  I will readjust as they wear in.

My throttle cable is rubbing where the sleeve mounts above the cable connection at the carb.  It was hanging up pretty badly.  Got some adjusting to do before I take another test drive.

 

Ken = Good luck with your brakes.......

 Graeme

One thing that can affect pedal travel is the piston diameter of the rear wheel cylinders. Usually rear wheel cylinders (piston size) are matched with the style of master cylinder (piston size) and front brake set up. Did you replace the wheel cylinders with the same size and did they work correctly prior to the restoration work?

In other words, if you had 1/2" cylinders and replaced them with 3/4" cylinders, you will have excessive pedal travel.

Have someone watch the front pads when you press the first pump, if they move a little and don't bind (hold the wheel firmly) then the excess fluid is actuating the rear cylinders and there's not enough to pressure up the system and lock the brakes. (sounds like this is a possibility)

It could be the rears aren't adjusted properly or it can be as simple as your mini never having the correct wheel cylinders on it in the first place. After all, it isn't the first time they've been replaced in 37 years is it?

I don't know enough about which Mini had what MC or WC or Caliper piston size etc to tell you what you should have, but there are others on here that have that information for the asking.

Worst case scenario is you overhaul the front brake system and have the same problem. You can still go to a smaller WC piston size to gain back some pedal.   dan

Thanks Graeme.

I am not having the success you are.

Got out there this morning, and bled the entire system twice. I'm using a pressure bleeder, which is the easiest way to bleed everything by yourself. Since it pressurizes the whole system, all 4 brakes should be grabbing. That's how I checked the adjustment of the rear brakes. With about 20 psi of air pressure, I could still rotate the rear brake drums. 1/4 turn on the adjuster and - stops. Bleed pressure, and it rolls. That's gotta be good.

So - the good news - I have new rear brakes, and they feel adjusted correctly.

The bad news - pedal is still squishy on first pump, hard on the second. It's not as bad as last week, when the pedal would go to the floor, but it doesn't seem right yet.

I think I'm going to re-build the calipers now, and install new pads and rotors. With the whole system pressurized, I could still turn the front discs. I'd love to get new calipers, but dang - those babies are expensive.

So - order parts today and start waiting again.

Crap.

Ken

The Circlips are fun - especially if you sandblast, prime & paint your backplates and then scratch 'em all up trying to get them on!  I was trying to put them on by pushing them.  It was much easier when some told me the get the center and one side on and use a small screwdriver to lever on the other side.  Popped into place much easier - even saw a tool you can make to put them on with, but wasn't worth it for 2 clips.

DKLawson - thanks for the post on the pics - my manuals lack good pictures and having its been almost a year since the car was dismantled.  Have pics in hand and will be checking my brakes this morning.

I tightened all my brakes up until they locked and now have a brake pedal.  The rear brakes when backed off did not effect the pedal hardly at all.  However, it did not take much backing off the adjusters in the front to start losing brake pedal.

I am going to bleed the brakes one more time to check for an other air and adjust the drums and I should be good to go.

Thanks for all the posts - while I adjusted the drums (or so I thought), I never thought that the wheel cylinder pistons were absorbing all the pedal stroke.  But it makes all the sens in the world.  An easy check was just to tighten them up and see ofI had pedal pressure.

 My only remaining question is should the brakes be bled with drums tightened up or drums loosened off.  I have been told that sharp pumps and holding the pedal pushes air bubbles out to the end of the lines....