Yes of course there is fluid between the front and rear piston and the front piston and the front of the master cylinder. And none of that fluid connects the front and rear systems by hydraulics. It don't matter what the safety valve does because if you get a leak in the system the half that leaks will pull air in everytime you pump the pedal whether you have the safety valve or not because it has a leak and will pull air in. Yes if you get a leak in one system then the press in the working half will push the safety valve over to turn the dash lite on. The actual working of the safety valve will block some of the fluid in the leaking side just because the valve moves over and the size of the valve will block much fluid in the leaking side but it don't block all the fluid and it don't really matter because it still has a working half. Take the safety valve out and it still works the same which is one side leaks brake fluid and the other side still works. Safety valve or not one side leaks fluid and the pedal will compress the spring in the M/C on the side that looses pressure until it mechanically hits the M/C piston and works the non leaking side so you have some brakes. You loose 1/2 the brake pedal but you still have some brakes. It only makes sense that there is no way the two systems are hydraulicy connected so that one side will still work if a leak happens in the other side. That's why they went to dual M/C's in the first place. What sense would it make to have dual system and then connect them hydraulicy ?? None because if it got a leak you would have no brakes at all so then you might as well just stay with a single M/C. Try asking yourself WHY they went to a dual M/C in the first place ? Ron

It don't matter if the valve did block fluid because the side it could block fluid in has a leak and is not working if the valve has moved over. You still lost half the brakes and the master cylinder will compress the spring in that side that lost fluid and then push the piston in the working side. You could let all the fluid run out of the leaking side and it will still have some brakes from the working side. If what your saying was true then how could I have bled my brakes on my dual system in my 63 when I have no safety valve in it ? Ron

You obviously do not understand compressibility and incompressibility.

I can also tell you about three years ago I noticed my brake pedal had gotten lower. I did not realize that my master cylinder was leaking at the back side and the rear resivoir went empty. But I still had brakes with a lower pedal of course. Ron

When there's a "leak" down stream past the safety valve, the valve plunger slides over to the side that has the leak (by pedal pressure). That plunger will stay there indefinitely until the light switch is removed to allow it to go centered again after fixing the "leak". This valve action keeps the fluid conection sealed between the two pistons in the MC so you have a good pedal for either front or back, whatever ones not broke.
Without the valve you basically have a single MC, thus easy to bleed.

No it does not. Just think if you get a leak before the safety valve. Say the line at the M/C loosen's up and one of the M/C resivoirs goes empty. That leak is before any fluid even gets to the valve yet you will still have a partial brake pedal because the other half of the dual brake system will still work. It will be a much lower pedal but it will still have some brakes. That's why they went to the dual system and why its like 2 separate brake system so one side will still give the car some brakes.

That valve does not seal the line when it moves over. It will keep a lot of fluid from running out because the physical size of the valve that moves is almost the same size of the passage the fluid goes through in it. It wont be a solid seal and some fluid can still get by it. If you get a leak before the valve like at the line coming out of the M/C the valve will still move over and turn the dash lite on because you lost pressure in the side that has the leak and the working sides pressure will still move the valve over. But the valve will have nothing to do with blocking any fluid if it leaks before the valve yet you will still have 1/2 the brakes working. Its made so you can run one side of the M/C completely dry but it will still have some brakes from the working side.

Just think about what happens in the dual brake system if it leaks at the M/C line so the leak is before the safety valve. It could lose all its fluid on that side so the safety valve will have nothing to do with the fluid loss but it will still move over by the pressure in the working leak free side. And that side that still has pressure will still let the driver have some brakes to stop the vehicle. It wont be great brakes but it will have 1/2 the system for an emergency stop. That's the whole concept of the dual system. Ron

Obviously I do. Ron

Boy,hope I don't have to ask a question about a caliper !!

You can talk about this till heck won't have it and that talk will solve nothing.

I really don't want to ruffle feathers, so do this. Open a bleeder and stomp on the pedal. I say it will go to the floor, because I know it will. GM, or Mopar, they all will do it.


I could be wrong. I'll buy the fluid for you, I pay via Paypal.
Let us know.

This is sort of like quantum physics, IF you DON"T test it, you will have dual brakes. If you do test it, well then you will know SOMETHING.

This bit of ignorance begs to differ.



What connects the front and rear piston in the M/C when you step on the brakes in a normally operating split system?

HYDRAULIC FLUID, there is no mechanical connection at this point.

Why does it work that way?

Because hydraulic fluid is incompressible and you get instant action on both.

What happens if you have a leak? Depends on which end but in either case neither one makes and pressure till the piston with the leak side bottoms out mechanically. Degrading your braking. UNLESS, you have someway to pinch off that leak (safety valve). Which lessens the degradation.

It's really very simple if you just look at the Federal Regulations for automotive brake systems published in 1967. Although almost all automotive manufacturers had adopted the dual master cylinder before that date, the actual Federal requirement wasn't adopted until that year. The following is the section that requires a dual master cylinder system and what the system is required to do.

49 CFR 571.105 - Standard No. 105; Hydraulic and electric brake systems.

S5.4.1 Master cylinder reservoirs. A master cylinder shall have a reservoir compartment for each service brake subsystem serviced by the master cylinder. (where brake subsystem means front or rear brakes) Loss of fluid from one compartment shall not result in a complete loss of brake fluid from another compartment.

The entire reason for the requirement for dual master cylinders was to provide emergency braking if pressure is lost in either subsystem and that's exactly what dual master cylinders do. Single master cylinders do not have that capability. In dual master cylinders if you have a leak in the front or rear brake lines the other will still be capable of stopping the vehicle. The requirement for this is stated here:

S5.1.2.1 In vehicles manufactured with a split service brake system, (front and rear systems) in the event of a rupture or leakage type of failure in a single subsystem, other than a structural failure of a housing that is common to two or more subsystems, (master cylinder) the remaining portion(s) of the service brake system shall continue to operate and shall be capable of stopping a vehicle from 60 mph within the corresponding distance specified in column IV of table II.

Because the loss of rear brakes may not be immediately noticed since most braking is done by the front brakes, a requirement for a pressure switch was adopted but this happened in subsequent years, not in 1967.

^^^^ AND how is this accomplished? With the safety valve!
What that says above is the mandate on having the new MC, not HOW it functions.

It is NOT a safety valve... according to the '67 Plymouth maintenance manual its a SAFETY SWITCH. It's actual designation is Hydraulic System Safety Switch. It's description reads as follows: "The Hydraulic System Safety Switch is used to warn the vehicle operator that one of the hydraulic systems has failed. .... As pressure in one system fails, the other system's normal pressure forces the piston to the inoperative side; contacting the switch terminal, causing a warning light to come on in the instrument panel, thus warning the operator of the vehicle that one of the systems has failed and should be repaired."

The switch does nothing other than warn the driver that one of the brake systems has failed. The MASTER CYLINDER provides hydraulic pressure to the other system when either the front or rear system has failed, NOT the safety switch.

BS

Dan,bet you're sorry you ask !!

You can call BS on a direct quote from a Mopar service manual if you want... however, its my guess the engineers who wrote it back in the day had a bit more knowledge on the subject. But have it your way...

The engineers wrote it as a very basic description for a mechanic. They did not get into hydraulic circuit design and theory in that write up. Which is why it's full function is not explained. All the mechanic needs to know is that if the light stays on, or it never came on, the problem is that switch.

And that's the limit of your understanding too.

Let me go back to the quantum physics. IF you want dual brakes without a "valve" DON"T test it.

If you want to solve all of this, test it, I'll supply the fluid.


I find this kind of astounding (but fun).

Exactly what we where told way back then. Just a basic reference.

And as I said before yes there is brake fluid between the two pistons in the M/C but they are not hydraulicly connected because there is a piston with seals in between them. Yes fluid is on both sides of the pistons but if one side leaks the other side will not loose fluid and leak. When the fluid in between the pistons is lost from a leak in that side then the M/C compresses the spring that's between the pitons or at the end piston so you still have half the brake system. Yes the pedal will be lower but you will still have some brakes and the safety valve has nothing to do with it. As I stated before you can get a leak right at the M/C line and that leak is way before any safety valve that turns the lite on the dash on.


And I agree with you about a leak as the piston will move and compress the spring and bottom out so you have a lower brake pedal but you still have some brakes to stop with that did not leak any fluid from its side. Yes the brakes are degraded some when you get a leak in one side but you only loose half the brakes. The pedal drops and is much lower but it will still have some brakes which is the half that did not leak. If the two systems were hydraulicly connected then you would loose all the fluid with a leak and then that would make no sense to even have a dual system.


In a normal operating system there is fluid between the pistons but the front and rear systems are still not connected by their hydraulic fluid because one side can leak all its fluid and the other side will still work with a much lower pedal since it has to compress the spring and bottom out the piston. The side that did not leak will still work and have all its fluid. And it does not have to have any valve in the system for the brakes to still work with half the system. Ron