What's the purpose of dual master cylinder?

[btrc]

What is the basic reason for a dual master cylinder?  I had always thought it was for safety in case a line broke but now I don't know. Coming back from BTTF I had a brake problem with my '37.  We were driving along and it sounded like we ran over something.  We decided to stop at the next exit to see if anything on the car got hurt.  When we pulled off on the exit ramp there were no brakes at all.  Luckily we were in the middle of nowhere and just coasted to a stop about 1/8 mile past the stop sign.  Turns out that the small (about 1/4") bolts that hold the caliper to the bracket on one of the rear discs (T-bird IRS) had somehow backed out and disappeared.  This let the caliper move up out of the bracket and a pad came out and got caught in a spoke of the wheel.  This busted alll sorts of stuff up.  
  The point is that when I lost this caliper I also lost all the brakes.  The front and rear systems are plumbed separately so I would have thought I still would have front brakes.  I didn't lose a drop of fluid.   A friend of mine had a line break in his totally stock Oldsmobile and didn't have any brakes either so I know it's not just my system.  
 So what is the function of a dual master cylinder??

Bob

[rooster]

Bob, I cant answer your question, but I would have thought when you did apply the brakes the proportioning valve would have repositioed its self still haing front brakes only, just a though. It a good thing it happened where it did.

[PeterR]

When you say dual master cylinder I suspect you are referring to a tandem M/C, that is the type with a single pushrod but two distinct circuits.

For the purpose of this description let's assume the M/C is mounted in the normal position on the firewall with the pushrod pushing in the forward direction.

The push rod bears against a piston right at the rear of the M/C.   This is generally known as the primary piston.    About halfway down the bore of the M/C there is another piston usually known as the secondary piston.

In normal use, when the primary piston is pushed into the M/C, the fluid in front of it feeds one circuit and also pushes against the back of the secondary piston.   This forces the secondary piston down the bore to pressurise the fluid in front of it and feed the second circuit.

If the circuit fed by primary piston fails, then the primary piston continues forward until it hits the back of the secondary piston.    The secondary piston then pushes fluid to its circuit in the normal manner.

If the circuit fed by the secondary circuit fails, then the secondary piston, the fluid trapped between the two pistons, and the primary piston all move forward until the primary piston hits the blind end of the M/C.   The primary piston then pushes fluid out to its circuit in the normal manner.

In either case the pedal sinks down until the piston serving the failed circuit "bottoms out", and if the capacity of the M/C is borderline, the pedal will hit the floor before enough pressure can be developed to apply the brakes on the remaining circuit.

This usually occurs if larger brakes are installed without also increasing the diameter of the M/C bore, or if a small bore M/C has been used to avoid the need for a vacuum booster.  

Also, many M/Cs are designed to provide more fluid to the front brakes to take account of the larger front pistons.   If during plumbing the M/C the front and rear lines are connected to the wrong ports, then the amount of fluid available to operates the fronts can be just adequate under normal conditions, but runs out of stroke when the other circuit fails.

[OzRod]

http://auto.howstuffworks.com/master-brake1.htm

Sort of like this. Scroll down and click on the red arrows.

[btrc]

Thanks for the info Peter and that was a great Web Site Ozrod which confirmed exactly what Peter said.  I've got a M/C from a T-bird so it should be matched to my brakes but the system does seem only adequate.  It's something I have to do a little work on.  I suppose that when the rear took all the fluid that there just wasn't enough volume left for the fronts.   I'm going to double check my linkage to make sure I have full travel.

[model a vette]

Probably the best master for this situation is a '68 to '82 Corvette master. It is designed for 4 wheel discs with 4 piston calipers so the volume is pretty big.

[PeterR]

If all the components have been sourced from the same original vehicle then the problem is likely to be either,
1.  The master cylinder is not getting full stroke, or
2.  More fluid is required than necessary due to mechanical inefficiencies.

To test for full stroke
Have an assistant open a bleeder on one rear wheel and also one front wheel then slowly depress the pedal.   It should come to a stop by hitting the end of the master cylinder stroke before touching the floorboard.    Confirm the assistant has closed both bleeders before allowing the pedal to rise or air will be sucked into the system.

To test for other losses.
Check the wheel bearing adjustment.    If loose, the wheel/hub/disc assembly will wobble pushing back the caliper piston and then require more forward movement before pressure is applied to the pads.

Check for disc warping.   Any runout in the disc will push back the pads causing the same situation as loose bearings.

Check the flexible hoses.   As fluid pressure increases the hoses swell and contribute to "lost pedal movement".    Even hoses in good condition consume a surprising amount of the total fluid displaced by the master cylinder, and if the hoses have gone soft then the swelling can consume enough fluid to cause trouble.    A non-scientific assessment of hose condition can be made by holding the hose between the thumb and forefinger, while an assistant applies and releases full pedal effort with the engine running to provide vacuum assist and feel if the hose bloats.

Confirm the front & lines a connected to the correct ports on the master cylinder.

After checking all of these, test for improvement by opening a rear bleeder then depressing the pedal.  With a bit of luck even with full effort the pedal will be clear of the floor and you will have braking in the front circuit. Confirm the assistant has closed the bleeder before allowing the pedal to rise or air will be sucked into the system.

If the problem still exists I am out of ideas.  As has been suggested you can install a larger diameter master cylinder ---at the expense of increased pedal effort for normal braking.   However if all the components have been sourced from the same vehicle this should not be necessary.

[Mikej]

That was a great tip on the wheel bearing and warped rotor. I would think that you would have noticed the pedal going to the floor or close to the floor before but if one circuit was effected maybe not.