A few years ago I pulled every drum spliter I saw in the local yard.Most came from Darts.I also took 3 or 4 PP valves for when I went discs.My buddy ran a yard so I could get small stuff free or super cheap.Only discs required a P{P valve to the rears.Rocky

You break a front line, say hello to the safety sand at the end of track. The only way a dual master "works" is with that valve. Undo a front line, see if you have brakes, then do a back line, let us know how that works. Make sure you don't run into anything.

As I said, you need to mimic a 68 Fury (or similar) dual reservoir setup.
It's not rocket science. It's not even a hard think to solve.

I know I still have brakes with a front or rear line leaking. I bled the brakes myself when I built the car and with a front or rear bleeder wide open I still had half my brakes. On an older all drum brake system all that valve is for is to tell of a leak in one of the systems. If a front brake line leaks the pressure in the rear will push the valve over to turn on the brake lite on the dash on. The front and rear are still separate and the front line would still leak and the rear system would not. There is no metering valve in an all drum system and the prop valve is in the rear line. The only valve I could put on my all drum system other then the prop valve in the rear line is the safety valve and all it does is turn on the dash lite. The front and rear are still separate even with the safety valve. I wont run into anything as I still have brakes with one system leaking. Ron

Correct. Dual master cylinders are designed to work if either the front or rear brake systems have a problem. That's why they came into use... it was a safety issue. With a single pot system if you lost pressure anywhere in the system you would loose all braking. When you introduce front disks into the equation you need to include a proportioning valve to keep the rear drums from locking up before the disks under hard braking. Again this is a safety issue for control under a panic stop.

Some systems use a combination valve which is really just an enhanced proportioning valve. It will do the job of a proportioning valve but will also have a built in metering valve which keeps hydraulic pressure away from the disks until the rear drums have a set amount of pressure. This keeps the front disks from locking up too soon especially under wet or icy conditions. It also includes a 10 lb. residual valve for the rear drum brakes which keeps pressure on the drum system to reduce pedal travel before they engage. It will also keep fluid from draining back to the master cylinder if the master is located lower than the drum's pistons as it often is on street rods when the master is located under the driver's floor.

A dual master cylinder system will function without a combination valve or proportioning valve and with one system leaking. The addition of the proportioning valve or a combination valve just tunes the braking system better and increases safety under panic braking conditions.

^^^^^ Totally wrong, there is a hydraulic connection between the front and rear piston in that master cylinder. When a line goes out the piston slides over in that valve to SHUT OFF the broken line allowing partial brakes, it's not a glorified light switch. Granted you feel something bleeding your brakes (the reason you bleed the front and back at the same time) but get out there running 25 mph or more see how you stop. Back in the early 70s mechanics from our Dodge dealership attended Chryslers brake school, they explained this and other things in detail to us, but I guess those teachers where wrong.

Mr. cudaman1969 is correct, which is why I brought it up again (and sort of got verbally nudged about it). With just a dual master, of any sort, you don't have dual brakes.

Open a bleeder, front or back, and the pedal will go to the floor, and it has to. There has to be a connection internal in the master. It is the valve, what ever kind, to shuttle over and cut off the non pressure leaking side.

This valve, usually only works when brakes are pushed relatively hard, not like when you are bleeding.


If you loose one end of your braking, pedal effort goes WAY WAY up to stop. This I know, from a '95 Dodge truck.


I don't think anybody is trying to be difficult about this, it really is just to inform people. I learned this the hard way on a street rod, years ago. It is not common knowledge, but should be.

I have to politely disagree. The front & rear systems are really not connected hydraulicly. They have fluid between the front and rear pistons and when you brake the piston in the M/C pushes against the fluid pressure in between the pistons in the M/C. But if you loose pressure from a leak in either system the spring between the pistons in the M/C will compress if you loose fluid from a leak in that half and the spring will compress until it hits the piston and still applies the half brake system you still have. But the pedal will be lower because you lost the fluid between the pistons and the pedal drops when the spring compresses. Same if the other half looses fluid from a leak it will compress the front spring and still have the other half the brakes. Even on an old all drum system like on my car if you have the safety valve on the car the front and rear systems are still not hydraulicly connected. Each system has fluid pressure on one side of a piston in the safety valve to move it over with a pressure losss but they are not connected by hydraulic fluid. Ron

I also went to many Dodge schools on all the cars systems as I worked at a Dodge dealer for 24 years as a tech and shop foreman. Not trying to be smart as that's not why I come here. I posted that pic of the M/C so you can see the internals. And the safety valve on the older systems is a glorified lite switch to warn drivers who don't know better they lost part of their brakes. Yes a piston will push a valve over in the lite switch and all that switch does is push a pin to make an electrical connection for the lite. Both the front and rear bake systems have fluid pressure to the lite switch but they are on different sides of the valve and have seals on the piston that moves but each system only has fluid on the its side of the valve. They do not connect their hydraulics at all. I actually have A 68 Dodge manual that says that but I would have to take a pic of it and then load it into my computer to post it which I don't feel like doing right now. But I can post it tomorrow if you like. And I mean that in a nice way as I don't mean it to be smart. It can also get confusing when we get into newer cars with more control valves on them but I am talking about the basic systems of the mid late 60's and 70's. Ron

Cudaman 1969 now that I think about it I believe I know what your saying. That's because when the valve moves over and turns the lite on if one system leaks the valve piston actually blocks a lot of the fluid flowing in the side that has the leak since it slid over into its fluid path. And what happens is when you fix the leak you have to bleed the system that did not leak so that the side that did leak will have pressure to push the valve back over into its center position. And it will only do that after the leak is fixed and that system is bled enough to have enough pressure to push the valve back to center. But the dual systems are still not hydraulicly connected as they still have the piston in between their fluid. Ron

Ron, just trying to make it safe, since there's fluid between the two pistons ( in the master cylinder) it's hydraulically connected.
Corrected

No if you open a bleeder the pedal wont go all the way to the floor on a working system. Course if you did brake repairs and have not bled the system to get both sides working then yes when bleeding brakes on a system that had both sides open the pedal may go to the floor until you bleed both sides so both are working normal if a leak happens. The pedal will go way down but it will still have 1/2 the brake system if a bleeder is opened on a working system. It will stop but it will be much harder to stop of course. The pedal will still be very low but not to the floor since 1/2 the brakes will still work. That's the reason for the dual system naturally so if one system leaks it still has some brakes the stop on. Ron

Many years ago I had the hose on the left caliper blow on my 74 Dart SE.

No front brakes at all, but I did have some rear brakes. I would not classify it as having had 100% of my rear brake capacity. But then again, how would I have known? The pedal went almost to the floor before anything happened.

It got me back to the base and I fixed it later.

383man has it correct. the piston in the master will stop when it "stacks up" against the piston with the leak. this will feel like it's to the floor, but is just the "stacked" piston hitting the end of the master bore. this can be observed when bleeding a dry system. you always start with the caliper/cylinder farthest from the master, usually the right rear, unless you have a cross system deal, or anti lock system that says otherwise. press down lightly and measure where the pedal ends up. bleed both rear brakes until there are no air bubbles using your favorite method. when satisfied, lightly push down and measure. it will seem and feel like you are on the floor, but in reality, you will be higher. the amount depends on the particular system, but higher it will be. bleed the front, and when done, you have the pedal you need, at the height you want. if the system were in fact connected, you would not lose your brakes, then look in the master and find one cavity full, while the other cavity is empty. i know it doesn't sound correct, especially when playing with today's masters that are different, in the fact most [if not all] have only a 3/4 wall dividing the two cavities, and a low fluid light warning when the fluid gets to a certain level, so as to not have a cavity go dry when coming to a stop or accelerating, washing the fluid over the dividing wall.

^^^ good stuff. I learned something today.

I'm not sure we did. The one way to settle this is to have two people without valves go open and bleeder and see what happens.



If it did work without a valve, then look at this scenario. Back brakes set up very tight, front loose. You push on the pedal, rear makes contact, and no further pedal movement, no front brakes applied. (Which is why there is the internal connection)


But, this should be a learning topic, but until we have some actual testing, we won't know.


I have done this, seen others, which is why I am reasonably sure all dual systems came with a valve. And not just to light a light.

I will say no more.

Probably best to sum it up, let's start with just drums...

Simply put, in a simple dual MC system there is 2 separate hydraulic circuits (front and rear).

When there are no failures in the system, and the brake pedal is applied, the primary piston moves to a point where it starts building pressure in the primary circuit which is also applied to the secondary piston and the secondary circuit builds pressure.

Now say there's a failure in the primary circuit. No hydraulic pressure, but after a certain amount of pedal and primary piston travel, the primary piston mechanically acts upon the secondary piston and is able to build pressure in the secondary circuit.

A secondary circuit failure would be similar and have to reach a mechanical limit before the primary circuit can build pressure.

Now let's add a pressure differential valve to the system. This will activate a warning light if either circuit loses pressure. I can't find anything saying this valve does anything else.

Now let's add front discs to the primary circuit. Since the systems work differently (discs vs drum), there's a lack of balance in the system. It's not really much of an issue until an emergency braking scenario.

Adding a proportioning valve (manual or engineered valve)to the rear circuit will fine tune the balance between front/rear systems. This reduces the chance of the rears locking up under heavy braking.

And finally the metering valve in the front circuit. The metering valve closes off pressure to the front disc brakes until a specified pressure is developed in the hydraulic system. This allows pressure to force the back brake shoes to overcome retracting spring pressure and move into contact with the drum. Pressure beyond this opens the metering valve, sending fluid to both front and rear brakes.

The best dual MC and disc conversion has everything above, but at a minimum you want to add the proportioning valve which is what I suggested previously.

Corrections welcome, but I think I have it summarized.

Edit: added some pics to help follow along

Well, I said I would say no more, but I just find this fascinating.

I put pictures of the actual valve you have shown in the second figure, a couple of years ago here in this forum. Can't find them now, but this is the same subject.

Look at the piston that actuates the light, move to the right, it cuts off the input to the back brakes. To the left, cuts off input to the front. But, when it does cut the flow off, the pedal can still be pushed to actuate the "other" end of the system.

Again, we need someone without a valve to open a bleeder and tell us what happens.

I am really sure about what will happen, and if we could post that result here, actual data, we can all positively learn, eh?

My god, have you people never pulled a master cylinder apart? You can show all the pics you want but they dont even show clearly what's going on. THERE IS BRAKE FLIUD BETWEEN THE FRONT AND BACK PISTONS IN THE MASTER CYLINDER. The very reason you "bench" bleed before installation. If you lose that hydraulic lock for whatever reason there's nothing to push the foward piston until the contact by the rearward piston (no mechanical conection with the brake rod till it bottoms out) which will give marginal braking at LOW speed. Without the safety valve, fluid and pressure are lost every time you pump the pedal. For the ones that don't believe this fine, do what you want, it's your life. The newbe, put the safety valve in the system, to hec with the light.
Something about a water trougth and a horse.

Cudaman, you are preaching to the unenlightened.

I understand you, if no one else does.