Classic cars forum & vehicle restoration.

[rcx822]

Hi again!!

I've read some manuals on how modern(ish) dual circuit master cylinders work. From what I gather there are two cylinders in series to each other down the length of the master cylinder. One in front of the other, so from the outside looks like one long cylinder.

What I'm not clear on is how the brake pedal force is able to act on both cylinders at once. If both cylinders were rigidly connected to the brake pedal then the cylinder with less slack in it will generate pressure first, stopping further pedal movement, so the second cylinder won't do anything.

The manuals I read seem to be saying that once the first cylinder is pressurised then the pedal force acts through the first cylinder and onto the second.

But then what happens if there is total pressure loss in the first cylinder? For example, the brake circuit has a major leak resulting in total pressure loss. Will then pressure ever get applied to the second cylinder? Or does the brake pedal have to move to the end of the first cylinder's travel in order to apply pressure on the second cylinder? That could be more travel than the pedal has?

I also read about the "bias bar" pedal assemblies that custom (and race?) vehicles like to use. They have two master cylinders in parallel (so to say), and a bias bar pushes on them. This setup apparently meets IVA requirements (so the vehicle can be road registered). But I've heard people saying that total pressure loss in one cylinder means that no pressure gets applied to the second cylinder (because the bias bar moves on its pivot when this happens?). So it seems there's little safety gain from having two cylinders on a bias bar and maybe this setup is used more to allow balancing braking between front and rear wheels.

[Penman]

Hi
Have you seen this?
http://www.tegger.com/hondafaq/mastercylinderreplace/howworks.html
_________________
Bristols should always come in pairs.

Any 2 from:-
Straight 6
V8 V10

[rcx822]

[PAUL BEAUMONT]

Hi rcx822, it depends on why you ask the question - is it out of curiosity or because you want to use or modify a master cylinder. I worked for Lucas Girling in the early 1980's and among other things worked on the master cylinder for the SAAB 900(I think that was the model) We knew the tandem master cylinder as the "double AS" (American Standard - that the Honda Article alludes to) These things are simple but cleverly calculated dependent on how the brakes are set up - again as the article alludes to. The SAAB had an unusual "X" split. Essentially the output comes from balancing the springs and cylinder bores to deliver correctly proportioned pressures. Your surmise is quite correct - if one system fails the other MUST not. This is achieved by allowing the first piston (that is normally cushioned by fluid) to push through mechanically. The driver will experience excessive pedal travel, but again the system must function correctly on one circuit without running out of pedal travel. Hope this helps!

[rcx822]

Thanks, together with the article that explains all. So if there is total pressure loss there will be excess pedal travel but the other cylinder will generate pressure.

Why I ask. I am building a buggy. I've got the entire brake system from a Suzuki Samurai. I've been debating between race type pedal box and using the original master cylinder and servo.

However my decision may be forced because I am converting the rear axle from drums to calipers. The caliper pistons probably have four times surface area than the original drum cylinders, so I think I need to go for the race type pedal box after all to get the system to work.

When it comes to difficult technical questions I've found that the theoretical and technical knowledge on this forum is way better than on the mainstream forums, so please forgive that my project isn't quite classic!