I have witnessed one racing brake failure and recently heard about another one recently that is making me wonder about our brakes and how they work.

Based on the brake system schematic in the Haynes manual (page 110), it appears that our system has a "front circuit" and a "rear circuit".
It looks as if the front & rear connect to separate chambers in the master cylinder.
Each chamber is driven by a separate piston, but connected to the same shaft (they cannot move independently).

So if the above is true, then if you snapped off a front brake line (what happened to my brother) the front circuit would be dead, but the rear circuit should still be able to build pressure and stop the car (although not nearly as quickly).

Same goes for breaking a rear line, you should still have front brakes.

However this makes me ask two questions:
1 - How can you possibly put a brake bias valve into a system like that?
I thought brake bias valves worked by allowing you to divert more pressure to the front and less to the rear (or vice versa), but if the front & rear systems are completely independent, then this would not work.

2 - If one system remains operational if the other fails, then WHY does the pedal go to the floor if you open one bleeder valve?

Confused

It's not a brake bias valve.
It's a pressure spring that is adjusted so the rear brakes don't come on until a specified pressure is achieved to prevent the car from spinning.
Eric knows the exact pressure but it's somewhere between 500-750 psi.

For all the real racing brake setups I have seen, there are two independent pistons and reservoir tanks. The pistons are tied together at the pedal.

You're a little backwards. The thing in the rear is a brake pressure limiting valve. So pressure to the rear calipers can't rise above a certain level.

But to the original questions, I agree about the brake bleeding thing. I haven't noticed or heard of a situation where the dual circuit brake system has acted like everyone thinks it does. Although I can't say I've ever tested how much locking force is on the front wheels when a back bleeder is open (for example). I'll have to explore this next time I'm doing brake work.

Check out the Girling calipers on a 70's volvo p1800.
That was a common racing upgrade....4pistons, two circuits....quite cool all in one caliper...all about safety.
Bleeding is like doing two cars really..

Oh, they fit a 914.

Well the haynes book states the following:

"A dual master cylinder reservoir delivers fluid to the respective halves of the cylinder which operate independently of each other. If one half of the system fails the braking system remains effective using the remaining half; on U.S.A. models a warning light indicates this failure"

So it says that the each piston in the MC provides pressure to only one side of the system.
And if there needs to be a light to indicate failure, then it most not be completely obvious to the driver when one system stops working.

So why does the pedal go to the floor when opening just one bleed valve?
I will have to do an experiment next time I bleed the system...

Still doesnt explain how you could put a bias valve on this system if you wanted to.



Thanks but have to use stock brakes: Vintage racing...

A "bias" valve is more accurately known as a proportioning valve.
It reduces the rate at which pressure rises on the downstream side of the valve.
It has no influence on the other brake circuit. All it does is change the rate of increase on one circuit.

The factory rear pressure regulator essentially functions as a proportioning valve. The factory description indicates some more sophisticated operation from a unique valve arrangement inside, but my experiments don't support that. IMO, either the idea was flawed or the parts no longer function as intended due to wear.
The large spring behind the valve is a source of many perceived problems by 914 owners. When the spring is set too soft and collapses easily under increased pressure, enough fluid is displaced that the brake pedal continues to move to the floor, preventing one from achieving optimal braking.

By far the best way to gain control over front to rear bias in a race car is to install a dual master cylinder brake system with an adjustable balance bar between the two m/c's.

Chris,

Thanks for the info.
I looked into the porportioning valves at Wilwood, and it appears that there are two different kinds.
The first kind is exactly like our stock proportioning valve, but is adjustable from inside the driver's compartment.
Model # 260-10922

It is less of a true "proportioning" valve and more of a "limiting" valve since it does not really regulate front vs rear "bias" but just limits rear pressure to a preset level.
It does not interface with the front system at all.

The other kind was what I was imagining. It has TWO IN ports and TWO OUT ports.
It is a "true proportioning valve" and requires the front and rear systems to come together in the valve body.
Model # 260-11179

I like the dual MCs for the bias adjustment, but they still might not save you in the event of complete failure of one circuit.
My brother has that very system, and when the front system failed (fitting snapped off of the caliper from hitting road debris), the front MC bottomed out completely and left no travel on the pedal to push on the rear MC. We believe he had too little rear bias adjusted in which made the problem worse so it has since been adjusted. Hopefully we will never find out if that remedied the situation.

It is obvious. Completely, utterly, perfectly obvious. The light starting to flash is as redundant a warning as I think I have ever seen, in fact.

Since one of the two circuits does not build any pressure, it does not contribute to the pedal's resistance. So you wind up pushing the pedal twice as far with half the effort to get the same clamping force on half the number of brakes. The pedal instantly goes very soft and does little to slow the car. And you swear at the light because it's really very obvious exactly what happened...

--DD

HA! So it sounds like you have actually experienced this. So this system redundancy actually works?

That is good to know.

The second failure I read about was in a Subaru BRZ on the track.
The driver had just changed pads, and apparently didnt torque the caliper back in correctly.
the caliper came completely loose from the mounts and was rattling around in the wheel.
When the brakes were applied there was absolutely zero brakes.
In a modern system, I would not expect that to happen.
The ABS pump should have kept increasing pressure until the remaining brakes started working (or until you blow the pistons out of the caliper that no longer has a brake disk to press against)
http://www.ft86club.com/forums/showthread.php?t=56352

FWIW, the PO put in the cheap 19mm MC, I forget the brand.

One day, I discovered a puddle of brake fluid under the car in the vicinity of the MC. Pedal feel was still relatively normal and the brakes seemed to grip. I topped off the reservoir and very gingerly drove the 3 miles to my mechanic's shop. No drama and the brakes felt pretty normal.

I presume that's consistent with the rear circuit failing?

Again... I've "always" found the factory pressure regulators to work as described.

I still don't see how that can be described as or likened to a proportioning valve. The operation is pretty clear in the factory manual and I've disassembled quite a few and found them to be quite simple. At a set pressure, the micro switch shuts fluid off to the rears. As pressure equalizes in the chamber they come back in.

I have recommendations on how this valve should or shouldn't work in a race car but I'm not going to post them here.

I do not recommend the factory pressure regulator be removed in any street driven car.

I wholeheartedly support this. I removed the rear gizmo, put in a T and locked up the rears. The tail spin wrecked my car and a friends face (a little).

https://www.youtube.com/watch?v=hnTmBjk-M0c