Reason for tandem master cylinder for breaking 6

[frankiee]

I have a question
With a duel master cylinder, one for back and one for front, I thought this was for safty. If a line breaks, you have the other circuit to brake.
It does not work.
Am I wrong in thinking that its design is for a safty reason?
I have had several brake lines break when I was young with old cars, salted roads in Ontario, and a heavy foot.
I find that when a line breaks, my foot goes to the floor: period.
come to think of it. If a person forgets to pump up the front calipers after a pad replacement, same thing - foot to the floor
In my head I can not see why it would not work.
2 separate cylinders to front and back.
They do have an internal seal, don't they??
Signed
Confused

 

[evelrod]

Well confused, your experience parallels mine. "Safety" brake systems were the big deal in the late 60's. My experience with failed master cyl was not as expected, to be sure. The only car that actually worked when a rear line wore through was an old Volvo. They have the brakes set up diagonally...I'm not sure that explains why they worked and the Fiat did not. Just relating my experience.

On our race cars, the dual master setup is for balancing the brake performance, front to rear and not necessarily to preclude total failure. If one of the masters, either front or rear, were to fail as I go into a very high speed corner...well, let's just say I'd rather not find out.

Rod

 

[patdaly]

While the pedal will go to the floor with the first push, subsequent actions should bring some pedal back. If you look under the MC, you will find a balancing valve, typically where the brake light sensor is. There is a plunger which remains centered during normal operation. When one or the other side loses fluid, the plunger moves and activates the brake warning light. This is where the fluid from the first pump goes.

To be sure, the braking action will be terrible, especially if you happen to lose the fronts......but short of a panic stop, you should be able to safely stop the vehicle.

Rod, you could have had the same thing I ran into, the rears were already not working. Since they really do very little in overall braking, I hadnt noticed them, and when the fronts lost a line, needless to say it was interesting.

 

[evelrod]

Pat, I came close to that in a company truck, 1983 Chevy 3/4 ton with hydrovac servo. Rear brakes had failed due to bad axel seals (typical for that era Chevy) when the power steering pully broke. I was prepared for the hard steering, buuuuuuut...---picture 10,000 lbs., downhill, approaching freeway interchange/on ramp with a stoplight !!

Rod

Oh yeah, I made it, just!

 

[CESSNA1]

FRANKIEE: I think it is more of a marking ploy. My experience parallels everyone else. There was a big push for safety after Ralph Nader's book "Unsafe at any speed". That is when we paid for seat belts, padded dashboards, dual master cylinders, etc. I believe some were beneficial, but I saw no clear advantage to the dual master cylinder.

Regards
Dave

 

[frankiee]

"Unsafe at any speed"
I remember reading that book in 1978 when I was 13.
School bored me so I took a book.
Thanks for the replies

 

[patdaly]

Rod, yeeehaw! The worst thing about that was, since it had hydroboost and was only a 3/4 ton, it should also have been a diesel. You didnt even have the option of compression braking!

You, like I have lived a charmed life.

 

[magnograil]

Depending on what you mean by "old cars", the brake systems have changed a lot. Newer cars have dual diagonal brakes so a loss of one system still retains one front and one rear brake. Earlier systems had the fronts tied together and the backs on another system or all four on one system. Loose the front ssytem and the backs were about as good as dragging your foot.

 

[evelrod]

Discussions on "old cars" always interests me. I still find time to fool with my antique cars, usually while I am waiting for parts for the race cars ;-) !
I can say from personal experience with 30's era Buicks that the hydraulic systems of today do not differ greatly from those used in the 20's and 30's---often still identical, as in the case of my 37 Roadmaster...I used late model Dodge truck wheel cylinder parts !!!
For those of you interested in the history of brakes...

http://www.esceducation.org/about_esc/esc_origins.shtml

Rod

 

[Tmoose]

I think the front/rear VS diagonal evolution has resulted from the dominance of FWD, combined with the desireablilty of braking with both front wheels, or at least NOT braking with just one front. Since FWD front wheels are tied together with an axle, in non-slippery surface stopping one wheel stops both. With vintage RWDs with simple calipers both front wheels must be on the same circuit.

My Volvo 240 has complicated 4 piston front calipers with dual inlets, so each braking circuit picks up half of each front caliper, and one rear caliper. Nice, but complicated.

I can attest that Volvo's dual MC does work, as I was able to burst 1 rotted line in each circuit during a real hard stop. (still resulting in zero brakes, but the rotted lines were my fault)

 

[GregLocock]

Um, do you not have a differential in your front axle?


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Tmoose]

Does the differential reverse the braking torque at the other wheel ? I see that if one wheel was braked and locked on ice the other wheel in the other side of the diff would spin at 2X the input speed.

 

[GregLocock]

A poor man's ABS can be had by leaving the car in gear when braking. This makes it very difficult to lock just one wheel.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[trailingwire]

who takes a car out of gear when braking????

 

[evelrod]

In road racing, heavy breaking and down shifting at the same time means that ya just gotta take it outta gear at some point! I guess that mean I do it, to answer yur question.

Rod

 

[GregLocock]

I hit the clutch. That has the same effect. Ooh, I just realised, manuals only.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CentralCoaster]

It is for safety, and has nothing to do with brake proportioning. Head over to howstuffworks, their diagram will show you better than I can.

On a traditional front/rear mc, disconnect one brake line, you're pedal will move freely about halfway before it starts building pressure on the good circuit.