You are correct. They are definitely not 2". Don't know what I was thinking. The the external cylinders are 1" and the bores maybe ¾"? With the master cylinder bias bar optimized and the addition of the proportioner valve, the rear lock-up problem is now tolerable. I'm installing the 5/8" wheel cylinders this weekend and expect that will solve the lock up problem completely. 

Considering your 66/33 weight distribution and slicks, I would say you have indeed found the sweet spot, brake-wise. Had I known about the inertia valve I would certainly have considered it.

first, I guarantee that you do not have 2" master cylinder bores on a Mini. If you did manage to cobble something on that big, you wouldn't get enough pressure to the wheel cylinders to stop the car on the same day you applied the brakes. There are a range of rear wheel cylinders available, including the mentioned 5/8" size. You can experiment with the 9/16" size as well, but it is simpler/easier to use the stock Mini inertia valve that came on some models. I have had very good success using this unit and it can be made adjustable simply by changing the inclination angle, by bending the bracket. My car has a poor 66/33 front/rear weight bias due to the heavy Type-R Honda engine and only a 3.2 pound battery in the back, light weight rear swing arms and hubs, plus the front brakes are the later big discs with 4-pots and very aggressive pads. I too run on race slicks. I still use 3/4" rear cylinders and after a couple adjustments on the inertia valve angle, was able to get the right brake balance without using the twin master/bias bar setup. If the car stopped any harder, the rear wheels would get air. The nice thing about the inertia valve is that under normal/light braking, the rear brakes get full pressure and do their fair share in stopping the car.

... okay, more/less sophisticated than I am when it comes to breaking systems. I also noticed in the tech info a casual reference to "rear calipers" - may be a red herring or maybe that particular valve is for rear disk application. Beyond me!

I would say less sophisticated. The factory valve shuts off fluid transfer under heavy braking, which in turn stops the rear wheels from locking......

"Both master cylinders have a 2" bore".

this could be A Clue.

Thanks for the technical input. By adjustable proportioning valve, I took it to mean something akin to the standard proportioning valve common to all Minis, but with an adjustment screw (there have even been threads here about modifying a stock valve to make it adjustable.) Obviously the one you have is more sophisticated.

So it would seem. Technically speaking, it's not really a "proportioning" valve but simply a pressure reduction valve. According to the manufacturer's literature, the line pressure (in this case, the rear brake line) is momentarily reduced until the pressure between the master cylinder and the brake cylinders equalize. This is supposed to inhibit momentary rear lockup due to initial weight transfer. As I understand it, the balance bar between the front & rear master cylinders are responsible for the actual bias balance.

From Tilton Racing: "Tilton proportioning valves allow you to adjust the brake line pressure to a particular wheel or wheels. They are also used with single or dual master cylinders to give you a better balance front-to-rear under light or aggressive braking. The Proportioning Valve is installed in the rear brake line and allows you to adjust the rate of pressure rise to the back brakes. With an adjustable proportioning valve, you can slow down the pressure rise that occurs when you apply the brakes. Under braking, most vehicles transfer more weight to the front wheels. When this happens, the rear wheels lose some weight and can lock up if the pressure is too great too quickly in the braking process. Eventually full pressure is applied to the rear brakes through the proportioning valve."

...but to 'proportion', wouldn't it need to 'see' either the front system or at least the front brakes? To proportion, it would have to divert excess pressure from the rear brakes to the front? Without that relief, wouldn't all the pressure applied to it just be passed through?

I'm with the other guys:  reduce the diameter of the rear cylinders so that your dual master bias set up works as intended, and then you can get rid of the redundant valve.

The bias valve is on the rear brake circuit only. It receives pressure from the rear brake master that has already been reduced to it’s lowest setting, reducing it further as it passes through the valve on its way to the rear brake cylinders. At least that’s what it looks like to me.

l don't understand: why would one use a bias valve with an adjustable dual brake master? Wouldn't it tend to counteract the difference between your higher pressure setting of the front and the lower setting of the rear? Seems redundant to me.

I've got my new bias valve set to minimum on the rears and I'm still locking the rears up before the front. Damn. Note: They only lockup under hard, hard braking. I've got ¾" cylinders in the rear—just ordered a set of 5/8". 


Yep. The 5/8" cylinders require drilling on the backing plates to accommodate the locating pins.



I suspect I'm locking the rears because I've got 4 pot KAD disks up front and sticky track tires. Plus, my car weighs about 150 lbs. less than stock.

Both master cylinders have a 2" bore and as I've mentioned can be independently adjusted. I've biased the masters for max pressure on the front and min pressure on the rears and biased the proportioner valve for min pressure at the rear. I've backed off on the rear brake adjusters as far as they go, the pads and shoes have <5K miles, all of the hoses are either copper or stainless braid, the fluid is fresh and all the lines have been bled. Everything moves as it should; smoothly without any binding. All that's left is smaller rear brake cylinders. Yes?


I suspect Hagerty is familiar with aftermarket parts.

I think a better question is: does your insurer know that you are driving about with a random connection of race car parts and mini parts on your car's braking system?

You're fighting a symptom not the cause...

First thing to ask is WHY the rears are locking up - and as the others have mentioned, it's almost certainly a mis-match of components.

What calipers and wheel cylinders are you running, and what are the bore sizes of your twin circuit m/cs ?

Not knowing what calipers you are running and if you have a booster picking a rear cyl. would be a guess. Running 7.5 disc and 3/4 rear cyls. with large left tank and Cooper S right tank I had no issues with a full load of fuel in the dry. Add sand or wet surface and the rears locked. With low fuel load the rears were likely to lock as well. That was with single master. I now run the 7.5 s with twin boosted late master and 5/8 cyls. Good brakes with torque tube cross over assembly and no further issues due to fuel load or conditions. Trying to recall if there was something I had to do to the backing plates in regards the locating pins. Steve (CTR)

Have you checked the rear wheel cylinder size? Stock is 3/4" but you may need 5/8".

Having a dual circuit braking system w/ separate master cylinders for the front & rear circuit, and despite biasing the front cylinder to max and the rear to minimum, I continued to lock up the rear brakes before the fronts. This proportional valve should do the trick. [link]