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Mini brake conundrum

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DrEntropy

DrEntropy

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At wits' end over this:

A '72 Mini of Spanish origin, cannot get a decent brake pedal. First depression on the pedal it goes nearly to the board and an immediate pump brings it to where it should be.

So far:

New master and all cylinders, booster replaced, any lines looking at all questionable replaced. Flex lines new. Proportioning valve for the rear has been made "empty" and is just a coupling.

Have installed new shoes and drums.

Pumped GALLONS of fluid thru the system, absolutely no air in there. Bled "traditionally" at first (two-man operation, build pressure with pedal, release & close bleed screws rear to front), then the Eeze-Bleed, and in frustration rigged an A/C vacuum pump to the booster and bled it again... no joy.

At one point we speculated it was that the shoes were not bedded and the problem was caused by excessive mechanical "space" to take up, so adjusted all corners to be mechanically ~just~ at the point of lock-up. Bled system and came out with the same results.

Completely flummoxed and extremely frustrated! Any insight will be appreciated.
 
I had this issue with my mini at first after brake job. I finally used a vacuum bleeder and improved it greatly. I also jacked the rear of the car up so the air bubbles collect at the high point to be bled off. I would try more bleeding, because the other issue would be a bad master cylinder. (or a leak in one of the lines allowing air in).
 
First, the proportioning valve in the brake system is important. You need to replace it, rebuild it, or otherwise have an operational valve instead of the gutted one. Without one fitted the car will have a tendency to lock the rear brakes prematurely and this will cause the car to spin.

It sounds like everything is "new". I am unclear from your description whether the front is disk or drum. If disk, make sure the bleed nipples are pointed "up". It's all too easy to mount the left caliper on the right side and have the nipples pointed down... making bleeding impossible.

When adjusting the drums, take note that the adjuster has 4 lobes on its contact surfaces. Make sure you are turning the adjuster in 1/4 turn increments ending with the "flat" being in contact, not a high point. Correctly adjusted means an elevated rear wheel can be spun by hand with audible drag and the wheel will stop in about 1 revolution. Anything more means the brakes are too loose. Loose means you will have the pedal travel to the floor on the first pump and be "normal" on the second rapidly applied pump.

Shoes are "handed". Take a look at the picture linked below for a LEFT rear brake assembly. Note the friction liner is relieved at the top of the front shoe and bottom of the trailing shoe. The right side will be mirrored. If you don't have relieved shoes, grind away a bit of the material on the shoes you have (to heavily chamfer the edge that will contact the drum first). Failure to fit the shoes as shown can (but doesn't always) result in a spongy pedal with excessive travel. You can bleed forever and never get a firm pedal.
21A1060KIT.jpg

Since the system has been completely empty, try the following.
Fill the master cylinder. Loosen the pipe connecting to it and work the pedal slowly until fluid is seen coming out of the loosened joint. Stop and slowly let the pedal back up. Tighten that first joint. Move down the piping to the next joint and repeat the process. Do this all the way from the MC to the wheels at each corner. Then try bleeding again.

I don't have an EZ bleed and have never thought about using one on a Mini. The proportioning valve mentioned above does not like sudden pressure applications. They tend to shuttle closed which prevents bleeding the rear wheels. This is particularly true on dry/empty systems. Instead I have a bleed hose with a check valve which I use to pump the brakes slowly for 1-man bleeding. You can do the same by replacing the wheel cylinder bleed nipples with "Speed Bleeders" which will be available at the local parts store.

To summarize, make sure all the parts are on in the correct orientation, then make sure the drums are adjusted properly. Crack the piping joints sequentially from MC to the wheels, allowing a bit of fluid to come out of each before tightening. Finally, bleed manually and slowly using little to no force on the pedal so the proportioning valve won't close off the rear wheels.
 
Thanks Doug. ISTR a thread about this some years ago, the mention of the chamfer on the shoes' material rang a bell. I believe that thread involved Tony, too. Don't know why I didn't think of it!

Will check (again) that we've got all the mechanicals correct. We took photos before disassembly, but this thing had been sitting static for over a decade and who knows why. Looks like a proportioning valve is going in as well. When the brakes work after the second pump, the thing doesn't exhibit any bad behavior but your point is well taken.

The Eeze bleed pressures can be lowered to accommodate the valve's "sensitivity" but will have two of us there to do the job "by hand" (or foot in this instance) anyway.

We've got shoes with NO chamfered lead ends. That's going to be "step one." I have high hopes for it.

We've been doing this stuff for decades, never ran across a brake system to give us this amount of grief!

Again, thanks Doug.
 
Sadly, Minis can either give you a totally pain free bleeding procedure or one that makes you want to shoot the car. I've experienced both.
 
Yeah, this one has had my confuzzled from the jump. Made in Spain and some things look subtly "different" from the BMC Minis I've dealt with back-when. It deserves shooting!

...but it's a client's car, sooo... onward.
 
I only know of one acquaintance who has owned an Authi. I know it wasn't roadworthy when he got the car and I never checked in with him to see if he got it sorted out.

Keep us updated with your progress. I'm sure you'll sort it out.
 
Small improvement with the chamfering, it is taking "laps" and raising the rear for readjustment. There was a two-flat adjustment on the first run, no doubt the shoes "centered" themselves on that go. We progress apace. Each successive drive seems to give it some space to dial out. Pedal is improving as well. Front drums are also being checked/adjusted.

Still considering the "larger" diameter MC, as moving more volume couldn't hurt.
 
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We've got the Forum, Steve (praise be to Basil)! My Mini experience is a lot less than the guys here, I'm a LOT more familiar with MG/Jag/Rollie quirks and such than Mini's. If Doug hadn't mentioned the chamfered shoes I'd be pullin' me beard hair out still-and-yet!

I even had to "Phone a Friend" WRT radiator issues; called Jim Boffo and bugged him a couple times. That was before the brake thing came into the picture.
 
Had to "radius" all shoes at the leading ends. Now the brakes seem fine.
 
I have never thought about the shoes being handed, I will have to check my rear shoes. (disk in front). Makes also want to check my TR3 shoes in the rear as they don't do much of the braking.
 
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