The Stovebolt Site Forums The Shop Area Engine Shop how much safer is a dual chamber master cylinder?

i'm working through the brakes on my 54 3800.

how much safety is gained from a dual chamber mc? i figure if you lose the rear brakes the dual chamber mc will enable the front to work or vice versa. in reality is this what happens?

the reason i ask is because, it does not look too easy to replace the single chamber mc on my 3/4 ton. the kits that i have seen are for the 1/2 ton. the frame rails are wider on the 3/4 and so the mounting kit does not fit.

i could fabricate my own mounting bracket.

The idea is that if the front or the rear line, or lines rupture or burst, the other will survive and give you enough braking power to stop the vehicle. Needless to say the chance of both going out at once is unlikely. With a single master cylinder, all you have left is the emergency brake.
To answer your question specifically, a dual master cylinder is twice as safe as a single master cylinder.
It is a good thing to get them to run well and attain highway speeds, but it is a better thing to be able to stop when needed.

I'm sure there's something out there that will work, eve without a kit. Your truck did not have power brakes when new, and it stopped ok then. Pedal pressure had to be designed so even a little weenie driver could stop it. Knowing that, check in with your local FLAPS for specs on bore diameter of your stock unit and bolt pattern to the frame. Find a dual chamber master cylinder with the same bore size, and mounting hole spacing and you'll be in it. Disk brakes were an option starting in 67 for GM, and standard in front in 73. I would suspect your best bet would be something in that 67-72 range, but it will take some digging in books to find out. Running brake lines is easy. You'll basically split the system at the master cylinder. May have to buy some line and adapters, but your FLAPS should have that too. You can make them neater if you cut and flare them yourself, but not everybody has the tools to do so.

Devin

Sorry guys, I am in a pedantic mood. If the failure of "one chamber" in a given period of time is a rare and random event, then the probability of a chamber-failure is most likely Poisson distributed.

This would mean that the probability of two failures in the same period is not quite two times (or, not quite 1/2 the probability). And, the probability of two failures in double the given period of time is also less likely than having the first failure.

So, I think that the dual-chamber master cylinder is more than twice as safe (in terms of total failure). And, you would, of course, replace the master cylinder very soon after a one-chamber failure.

I think this type of reply most likely belongs in the Greasy Spoon?

I agree with you Tim. If the probability of a single failure is fairly remote, the probability of the double failure is very remote, not just half.

There are some possible failure mechanisms that can result in total failure. When one system fails, one piston goes to the limit and the other moves into a position that it normally doesn't travel. If the MC is in poor shape inside this change in position could result in failure of the seal on the second piston. In any case when one half fails, the pedal will most likely go to the floor and a second stroke will be necessary to gain some pedal.

Simply, if a single master cylinder setup blows a fexible line, the whole system fails.
If a dual master cylinder setup blows a flexible line, you still have brakes on either front or rear, depending on which flexible line broke.

M W,

Fabricate your own bracketthis is a modified original 54 3100 bracket.

Lonnie,

Nice bracket and nice slide show and explanations/descriptions.

What year Corvette master cylinder will you use?

What will you have to do to add fluid when bleeding the system?

Did you need a new/different rod that goes into the front of the master cylinder?

Thanks,
Tim

thanks for the input. i'll see about getting a dual chamber mc that will work with little to no modification.

bending a flairing brake lines is no big deal but i dont want to start swapping out all kinds of parts. if it is reasonably safe with a well maintained single chamber set-up i may just stick with that.

tclMAN,

Just for everyones information, the MC in the pictures is a 1970 Mustang. It is a good MC to use for the conversion, but I wanted to plumb the lines on the frame side and not the trany side.

The MC I now have is from CPP. Any of the various vendors sell a Corvette style MC. They have ports on both sides, come in various bore sizes and the mounting holes are slotted to mount on a GM spread or a Mustang spread.

As far as filling the MC, I will probably relocate the original hole.

The rod is 3/8 diameter CR steel w/3/8-24 threads on one end. I ground a tapered, rounded end on the MC end. I bought the yoke from McMaster Carr as well as the rod.

The bracket and gusset are made out of 1/4 in thick plate.

Lonnie,

Thanks for all the information.

What year Corvette master cylinder?

Tim

Tim:
I agree with your assessment that a dual chamber mc is much safer than a single chamber, however, I not so sure that I agree with the application of the statistics you used to get there. It seems to me that academic solutions often rely on the old economics 101 qualifier "ceterus parabus", meaning "everything else being the same, or being constant, or being equal" (choose your definition). It seems to me that in real life things are never the same, or constant, or equal.

Also, there are two things changed between a dual chamber mc application and a single chamber application; the actual mc and the changed fluid distribution system (the brake lines). Quite a bit of the safety improvement is due to the changed distribution system, not the dual chamer mc per se. Looking at master cylinders by themselves, not considering the changed distribution system, I would think that in real life, if a particular dual chamber mc was in such a deterioriated condition that one chamber would fail, there is a pretty good chance the other chamber is not in much better condition and also subject to failure in the near future. So, (again ignoring the distribution system) I think both sides of a dual chamber mc would fail before two single chamber master cylinders would fail, one after the other, making the dual chamber mc less than twice as safe as a single chamber mc. (At least, my inner pedant thinks so.)

Actually, I realize that everyone is considering the dual chamber and it's distribution system as a single system and saying the dual chamber system is "more than twice as safe" as a single chamber system and I agree with that, but that's not as much fun as trying to confuse things.

So, so you see my feeble point or should I just shut up and stay away from the computer for a week?

Larry

Even if its only slightly safter, I think its worth it. Does anyone know if the kits classic parts etc have to convert to a dual cylinder will work with a 3600? I can't imagine they can be much different from the 1/2 tons.

It's pretty obvious nobody who has spoken up yet has ever experienced a failure of a dual-chamber system. The scenario goes something like this- - - - - -you'll have a normal pedal on initial brake application, then there's a "pop" that's felt, rather than heard, and the pedal will drop almost all the way to the floor before it gets somewhat firm again. Depending on whether the front or rear system has failed, you'll either experience a nosedive or a long, gradual stop, possibly with some rear wheel lockup and sliding. Any part in the system can fail, a wheel cylinder, a flex line, or even a rusty metal line. Once one system fails, the pedal drop is caused because the two pistons in the master cylinder must physically come together to move the remaining fluid, instead of having a hydraulic push against the second M/C piston. The vehicle will stop, but there will be a noticebale loss of braking effect. A single-line system will lose all the brake effect with a failure anywhere, however!
Jerry

I've experienced complete loss of brakes with a single circuit MC, it is no fun at all. A rear drum broke off and wobbled around which allowed the wheel cylinder to blow out. Really didn't even start as a hydraulic problem, sure ended with one though.

If there is even a slight chance that a dual circuit MC can reduce the pucker factor it has to be worth it.

Grigg

When I worked in garages as a kid, the first thing I did getting into a customer's car was stomp on the brakes. I had several fail right away because of that.

Also, I know personally at least two people that had complete failure with the dual system in a Chevette. I think it was probably in the design of that horrible little car!

One thing I haven't seen mentioned is the installation of a brake warning light for the dual system. I know that it is a complication because of the valves and extra plumbing involved. I think in a custom setup it might be possible to adjust things in such a way that only one system was working.

I bought the dual mc kit that is supposed to be a universal fit for 47-54 Chevy trucks and 53-56 F--- trucks. The brake line ports emptied right into my tranny linkage. I never could find a way to complete the hookup. As a result, my 52 GMC sat for months. I bought a new single mc and it is now back on the road.

the safest brakes are ones that are well maintained, regularly inspected and used prudently ..... in 50 years of driving single MC vintage drum brakes I've never had a brake failure of any sort - mechanical failures are rarely the result of vehicle design

Bill

thats what i thought.

ill redue the brakes, keep them well maintained and exercise some good judgement and i'll be fine.