I posted this question over on the "Under The Hood" forum but I thought I'd ask here too:

I'm working on the final analysis for Cale Kastanek's accident and I'm a little confused about the S2000's brake system. It's my understanding that the S2000 has a dual diagonal brake system. The right front and left rear are on a completely separate hydraulic system (other than the reservoir) than the left front and right rear. If brake pressure is lost in one circuit, the other two wheels should still have full braking and even ABS.

If this is true then why is it that you can easily push the brake pedal to the floor when bleeding a single caliper? This implies that a brake line failure would allow the pedal to go to the floor with little to no braking. That doesn't sound like a safe, dual diagonal brake system to me.

I had a brake caliper piston seal fail on me on the track and the pedal did go to the floor and it felt like a complete brake failure. I have it on video and it appears that engine braking may have been all that I had. I was in third gear at high RPM coming into a third gear corner so no downshift was required.

Can someone enlighten me on the S2000's brake system design?

 

Rob,
Here is the layout. It would appear the point of total failure is the master cylinder. Even though everything else is split, there is only one cylinder for pressure. At least that is what I believe. There is no diagram of the internal workings of the MC.

 

I'm pretty sure the master cylinder has two pistons. The reservoir is actually divided into two sections by a small baffle.

 

I think most of them are made that way. Evidently, it only helps with a minor leak. When my truck brakes failed, both front lines were cut. I "think" it is a front/rear system and I had nothing. (I will check on that).

I googled this:
To increase safety, most modern car brake systems are broken into two circuits, with two wheels on each circuit. If a fluid leak occurs in one circuit, only two of the wheels will lose their brakes and your car will still be able to stop when you press the brake pedal.

The master cylinder supplies pressure to both circuits of the car. It is a remarkable device that uses two pistons in the same cylinder in a way that makes the cylinder relatively failsafe.

 

I understand the theory of the dual diagonal brake system, but why does the brake pedal go from rock solid to full soft by simply opening one caliper bleeder (or having one brake hose blow out)?

 

You got me thinking so I did some research. Here is a link Dual Brakes Explained - the info is too long to copy. This seems to be a pretty good article - scroll down a ways to see info on master cylinders.

John

 

John,

Thanks for the link. It's a good article but it still doesn't tell me why I can open one bleeder and push the brake pedal to the floor. In theory the other two diagonal brakes should be getting full pressure from the brake pedal stroke, so pedal resistance should be halved (1/2 of rock solid?)

 

The test, then, would be to put a car up on stands, and open one bleeder. Then pump the pedal and have someone try to spin the other wheels. Then, you'd know for sure how it *does* work -- regardless of how it *should* work.

 

I'm sure you'd get some braking and you couldn't spin the wheels by hand, but the real question is how much braking do you get. A good test would be to open a bleeder and do an actual braking test. (not really suggesting anyone try it)

How do you go from rock hard brake pedal to push it to the floor no problem by simply opening one bleeder. Anyone that has bled their brakes knows what I'm talking about.

When I blew out my caliper seal last year my pedal went to the floor. I think I was lucky because I wasn't downshifting so I never depressed the clutch pedal so engine braking from 70mph in third gear helped tremendously.

 

I reread the document in the link I provided above and think I've figured it out. I believe the answer is in figures 23-13 through 23-16 (I can't figure out how to copy the info).

Here is a synopsis of what the figure shows -
1) Pedal is pressed
2) Booster increases force into master cylinder
3) Push rod forces primary piston forward
4) fluid in primary cylinder is pressurized to apply front brakes
5) Pressure in primary cylinder pushes secondary piston forward
6) secondary cylinder is pressurized to apply rear brakes.

Of course this example is for front front, rear rear braking system instead of a dual diagonal system, but I think it applies to our system.

Here is how I understand the system - the main link between the primary and secondary cylinders is hydraulic. The primary piston not only pushes brake fluid to the brakes it services, but it also uses the same brake fluid to push the secondary piston forward. The secondary piston then pushes fluid to service the brakes it is responsible for.

So, if one side of the system fails, you will have much more pedal movement until the gap normally filled with brake fluid is eliminated until either a mechanical stop is hit by the secondary piston, or the primary and secondary piston make contact making a direct mechanical link between the primary piston and secondary piston.

If the primary system fails, you will force brake fluid out until the primary piston makes contact with a stop that makes a mechanical connection between the primary piston and the secondary brake piston. From this point forward you're moving fluid through the secondary cylinder and the brakes serviced by the secondary piston will be engaged.

If the secondary cylinder fails, then they hydraulic link between the two cylinders will push the secondary piston forward until it hits it's limit and then pressure will go to the brakes serviced by the primary cylinder.

Based on the diagrams, I think this makes sense. It has been a long time since I've rebuilt a master cylinder so I can't be 100% certain. But, I think this might explain why we have so much pedal when we bleed our brakes, or lose one part of our dual diagonal braking system.

I would have to believe they make master cylinders this way for street cars is so that they don't have to do any additional clever work to maintain a balance between both braking circuits, the hydraulics will take care of this automatically.

I would think for performance driving we would want a direct mechanical link between the pedal to both pistons in the master cylinder instead of having the brake fluid pushed by the primary piston be the primary thing moving the secondary piston. This mechanical approach will make it more difficult to balance braking force between both brake circuits, but I think it would be much safer if one brake circuit failed. I wonder if this is how they do it in real race cars.

Disclaimer - I believe this info to be correct but I am not an expert on braking systems. This is only my analysis of the information I've found.

Thanks!

John