DUAL MASTER CYLINDERS ....... THE BIG MYTH?

I've been reading for years that you HAVE to use a 1970's Mustang dual reservoir brake master cylinder for safeties sake. They say if you blow a brake line, the brakes using the other reservoir will still work. I have a friend (who's opinion I hold in high regard), says this isn't true. It has something to do with there being 2 reservoirs, but only one piston working in tandom.
Does anyone have any first hand experience with brake failure-- Especially Mustang dual reservoir master cylinders?

 

Had a single master cyl. on a 63 T-bird.... rear brake line blew ....NO BRAKES AT ALL !! In traffic at 50 mph going into a traffic circle... One HELL of a ride !!! ...and an e-brake does NOTHING at 50 mph....( neither does shifting into low OR shuttin' down the motor...) had a dual cyl on a 73 cougar ... Same thing ...rear steel line blew .....pedal went REAL CLOSE to the floor ..BUT ... the car STOPPED !! I even drove it home about 20 miles with no problem except a low pedal ... a dual cyl. master is a NO-BRAINER !!

Jersey skip

 

I don't know about the Mustang dual reservoirs specifically, but the single piston control the flow of two reservoirs doesn't matter. With one chamber empty, the other chamber will still stay full. The full chamber will allow the brakes connected to it to still function, while the empty chamber will do nothing.
To demonstrate the point to your friend, get a dual chamber master cylinder, fill one chamber, and then bench blead it. You'll see fluid in the one chamber do what it is supposed to do (go out the hole), not flow back into the second chamber, or flow out the second hole (which would represent the brake line with the leak).

-Brad

 

The safety feature of a dual master cyl isnt so much the dual reservoirs but the 2 chambers working in the same bore.
If everything is working the way its suposed to the force of the pedal pushes on the chamber closest to it, and the hydroulic pressure of the first chamber will push on the second one.
If there is a leak in the 2nd system the piston that would pressurise the 2nd chamber bottoms out, so the 1st chamber is still sealed and will still work.
If there is a leak in the 1st system the 1st piston will bottom out against a built in stop that floats between the 1st and 2nd piston, so the 2nd system still works.
Since both chambers have their own brake fluid feed, it makes sence to keep the reservoirs seperate as well, but sometimes they are fed out of the same reservoir...

 

Dual master cylinder will not work properly without a proportioning valve.

 

A friend of mine claims the brakes went out on his 1992 ranger after a rear line blew. I said ******** you still have front brakes. He then went on to tell me that his ranger only has a single resivior. I don't really beleive him, not with all of todays modern safety features.

 

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Dual master cylinder will not work properly without a proportioning valve.

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I do not agree with that statement.
A lot of dual M/S systems will have them, because a lot of cars that have a Dual M/S, will also have discs in the front.
But there should be no difference in the amount of pressure between the 2 chambers in a dual M/S.

 

You may be right but the proportioning valve stops the flow of fluid to the end the line is blown.

 

No, that is still possible leadslead.
If you lose one chamber, you will use up some pedal travel before the chamber that is still good can build up pressure.
And a thick carpet, or badly adjusted brakes can use up a little more, and you run out of travel and still have no brakes...

 

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You may be right but the proportioning valve stops the flow of fluid to the end the line is blown.

[/ QUOTE ] No WAY man !! All it does is PROPORTION the fluid...that means "control" which pair of wheels get hit with how much pressure !! Blow a line on a car with a prop. valve and the master still goes EMPTY on that pair of wheels !! Hey... is this a test or what ?? Somebody is givin' you wrong information man ....

Jersey Skip

 

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He then went on to tell me that his ranger only has a single resivior. I don't really beleive him, not with all of todays modern safety features.

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The Federal government mandated dual master cylinders be installed on all new vehicles in the late 60's. I believe the year was 1968 but could be mistaken. Long before 1970 for sure....

.

 

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Dual master cylinder will not work properly without a proportioning valve.

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WRONG!! Ask the owner of Master Power Brakes. I did when I put brakes on my 57 Chevy. You do need a hold off valve.
I added dual master cyl, disc brakes and power brake booster to my 57. There is no porportioning valve. Its been working well for 5 years.

 

Proportioning vavles restrict volume and pressre on a single line.

Metering vavles restrict volume and pressure at a rising rate giving full pressure over time.

Combination valves can meter/proportion and have a floating balance valve that can trigger a warning light in the event of a pressure imbalance. These systems are usually designed to restrict flow to the low pressure side if there is a failure.

Dual masters are not completely failsafe. Catastrophic events can still happen and in the context of our hotrodding, if the system is not designed properly or poorly maintained the results can be terrifying if not horrific!

Metalshapes is dead on when it comes to how a dual master cylinder floating piston works. There are types that have multi-chamber single piston that do not use "proportioning" valves but are uncommon to us. Most diagonal bias systems use that type and usually have metering valves inline to the rear wheels.

 

Some questions for those who think a dual reservoir is a 'safety' feature. One half of the dual system stops working------How do you know? How does the average person know? Does the average person keep driving because 'the brakes are working'? Where is the 'safety' feature when the second half of the system fails?

 

If you lose one part of a dual system you will know...
The pedal is a lot lower all of a sudden and not nearly as effective.
It will scare you...
My only problem with a lot of these safety features is that eventhough the added safety is sometimes only marginal, some people will act like they are invinceble when they have them on their car.
Making them more dangerous to themselfs and people around them...

 

Manyolcars.... YOU'LL KNOW !! Pedal drops LOW --- INSTANTLY !!It even FEELS different ......

jersey Skip

 

Is everyone here using dual master cylinders?

I just bought a single master cyclinder set up (brand new) and this thread is making me worried to use it now on my project.

 

I had a car with a single resivor mc that broke a line. I can promiss you, everything since then has had a dual resivor. Life is already too short to go through that single resivor brake deal again. In fact, 4 wheel drum brakes are almost in the same cl*** as a single resivor. When I want to stop, I want the best stuff I can get. When I worked at the garage years ago, my boss always said" If it don't go, it is an inconvience. If it don't stop, it can kill you." Gene

 

YES on the Dual Reservoir master cyl's....they function as designed -you always have brakes on one end....I dont know why the poster had that problem-I have never personally heard of a total failure....
Now so as to beg for critique on the Proportioning valve use issue- I NEVER USE THEM.
I personally think they make a better Office Paperweight than a valuable ***et to MOST brake systems.
I suppose if you were running 4 piston Four wheel discs it may be of some value.....But I have built plenty of drum rear and disc front cars that stop just fine with none.

 

I was hoping the Disc/Drum discussion wouldn't spill over into this thread, but since it has...

It seems to me that even though most off the shelf Disc setups will be superior to most off the shelf Drum setups, If somebody is willing to spend the time and money, and selects the right parts, he can build a Drum setup that will perform really well.
( for the same time and money he could probably build a Disc setup that would perform even better...)
But just slapping a couple of Camaro or Volare Discs on a Hot Rod is not the last word in braking technology.
And you can fry the brakes on a lot of bone stock production cars ( if you are brutal enough with them ), whether they have discs or not...
In other words, running Drums doesn't automaticly mean you are playing Russian Roulette, and running discs doesn't make you invinceble...

 

I've kind of wondered this myself.

I always ***umed if you lost one side of the braking system the other would still function...until I went to roll my roadster out of the garage and down the steep driveway with the front brakes disconected. I reached in to push the pedal with my hand to stop the car as I usually do...and the pedal just went to the floor. Nothing. The car rolled out into the street...suprised me!

This was using a dual resivoir Mustang MC, and the pedal travel was not limited. Maybe pumping them would have got the rears to work. But I lost a lot of confidence in dual resivoir systems that day.

 

Guess I was wrong! Learned something here anyhow.

 

I had two UNSTOPPABLE incidents with my old '63 with single reservoir. One was when a shoe retaining anchor bolt broke, the other when the master cylinder seal went. You cannot imagine what it feels like to be coming up to a busy intersection at 40 MPH with a 4,500-lb. truck, and suddenly have NO BRAKES AT ALL. It's as if everything speeds up as it comes toward you.

My '40 will have a dual reservoir FOR SURE. I also will use residual valves in the lines so the pads are always close to the rotors. I'm guessing I won't have to "pump" that way. Don't know, I'm not a brake engineer. But I know this-- I will never, ever again use a single reservoir.

 

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I reached in to push the pedal with my hand to stop the car as I usually do...and the pedal just went to the floor. Nothing

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Then I'd say your rear brakes were out of adjustment!

If there's too much travel in the brakes, the M/C won't displace enough volume to let them grab.

 

Could be...but it was a perfectly working brake system when all hooked up, and if I'm **** about anything its the brakes.

 

I've also had a rear brake line blow on my single resevoir master cylinder car.....a 56 Pontiac. I'd been to Kansas City for the Cow Town Nats in about '83. Pulled off the interstate into a rest stop to take a pit stop. I used the 4 speed hydro to slow the car and listen to the pipes until I got into 2nd gear, when I applied the brakes and they went to the floor! Luckily, I wasn't pulling into a parking spot yet...just slowing down. The crowd was small and I coasted through in 2nd until I pulled the park brake and steered onto the gr*** at the end of the parking lot.
Drove it home on the park brake.
The entire mishap was all my fault too! If I'd given the steel lines a good look, I'd seen how rusty they were. After replacing the rustylines, I had no more trouble.
I've had single-resevoir master cylinders go bad before and I've always detected a problem before the cylinder "went out". Mine gave me a warning every time before failing completely. At stop lights, the pedal would almost imperceptibly "sink" a little before the traffic light turned green or the pedal would be a little spongy..maybe a little lower than usual, even after a brake adjustment. All warning signs of impending failure and a signal to get off the road and rebuild the thing or replace it.

 

Last time I ran a single cyl master I pushed the back wall of the garage out about a foot and a half, when a hose blew. Didn't do much for the work bench, or the grille of my 64 Falcon. I've run duals ever since. I like disc brakes too. Guess I'm just a *****

 

I had a single mc on my first hotrod.
Needless to say, it went out on an exit ramp around 70+.
thank God it was 2:30 in the morning, but the cop that was at the red light I blew through wasn't amused.
I installed a dual mc from a '69 Mustang and was very pleased with the increase in braking performance.
Be safe.
A dual mc is better than a single hands down.
My '54 truck has stock drums and a stock single mc. I plan my driving so that I have a bunch of room in front of me because the stopping distance on it is around a quarter mile at any speed (yeah, I exaggerating).
That's why I'm changing everything over to four-wheel discs this year.
I like to go fast, but what makes me real happy is being able to stop afterwards...
r

 

I took a wild ride in a 1960 sunbeam sportscar in 5 oclock traffic when the master cyl went out. Thanks to standard shift and an empty parking lot, I got out of that one. As far as duel MC go, they work just like everyone says. But the deal is if you're setting a pedal ***embly up in a car. Make sure when you mash the pedal to the floor WITHOUT fluid in it, it still has some clearance when it hits the bottom of the MC stroke. If it hits the floor before it bottoms out, you dont have enough travel to stop it with one side out of fluid. Keith

 

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I've also had a rear brake line blow on my single resevoir master cylinder car.....a 56 Pontiac. I'd been to Kansas City for the Cow Town Nats in about '83. Pulled off the interstate into a rest stop to take a pit stop. I used the 4 speed hydro to slow the car and listen to the pipes until I got into 2nd gear, when I applied the brakes and they went to the floor! Luckily, I wasn't pulling into a parking spot yet...just slowing down. The crowd was small and I coasted through in 2nd until I pulled the park brake and steered onto the gr*** at the end of the parking lot.
Drove it home on the park brake.
The entire mishap was all my fault too! If I'd given the steel lines a good look, I'd seen how rusty they were. After replacing the rustylines, I had no more trouble.
I've had single-resevoir master cylinders go bad before and I've always detected a problem before the cylinder "went out". Mine gave me a warning every time before failing completely. At stop lights, the pedal would almost imperceptibly "sink" a little before the traffic light turned green or the pedal would be a little spongy..maybe a little lower than usual, even after a brake adjustment. All warning signs of impending failure and a signal to get off the road and rebuild the thing or replace it.

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I couldn't agree more. Maintenance is a word that is not used around here. We talk about brake systems that have been in use for 40 years and if it fails we blame a M/cyl.for endangering our lives.

If you are driving an old car, the entire brake system need to be checked out...steel lines, hoses, wheel cyls, M/cyl etc. etc. That is the first thing I do on a new toy. If it is iffy...replace it. That's a larger step toward brake safety than adding a dual M/cyl. IMHO

It insults me when people insinuate that if you don't have disc brakes with a dual M/cyl that you are irresponsible and endangering innocent women and children.