Technical Tandem Brake Master Cylinder for safety

I imagine many of us run Tandem (some say dual) Brake Master Cylinder for safety on some of our old cars.

It is my understanding that the Tandem Brake Master Cylinder does not isolate the front and rear except for a few pedal pushes. Once one of the reservoirs is empty then the pedal goes to the floor. In today's new cars all the necessary isolation is handled in the ABS system.

The part that makes isolate the front and rear brakes is the differential valve that is found as part of the combination valve (some call it a proportioning valve). When the pressure from the reservoirs brake fluid is substantially lower on the downstream side of the combination valve the pressure differential will cause the valve to shift internally and isolate the leak and preserve the brake pressure in the front or rear that is not leaking.

The combination valve has other features such as metering, etc but to me at 72 yrs old, none more important than the differential valve that protects from total loss of hydraulic braking.

I have survived the total loss of hydraulic braking in 1980 in a stock 1950 Studebaker, but was fortunate that the hose blew while on an exit ramp that went uphill. Some downshifting and emergency brake use brought to a safe stop. Put a BB in the left front brake hose connection and drove on to the car show with 3 brakes.

Of my 6 old cars none currently have a combination valve. Only 2 have Tandem (some say dual) Brake Master Cylinders.

I would like to retrofit all of my cars with Tandem Brake Master Cylinders. and combination valves.
Today there appears to only be the common 70's thru 90's GM style in brass which are probably made by 1 manufacturer and I believe the GM style Master Power Brakes which appears to be black anodized aluminum.

P.S. The Wilwood Combination Proportioning Valve 260-11179 does not have a differential valve to isolate a leaking front or rear and when one of the reservoirs is empty then the pedal goes to the floor. The statement in their product description is not true "The combination block maintains full isolation between front and rear fluid circuits (call and ask them and they will verify this is false, I did today).
30 years ago all my rods had the Tandem Brake Master Cylinders. and combination valves because they were still used on new cars and good parts were plentiful.

I for one want to run my old cars 'til I can't (late 80's or early 90's) so I want to feel comfortable with my brakes.

If you will please respond with what you are running now and how to keep them safe far in the future.

Thanks - - -

 

Also, is there one valve applicable for a drum / drum system?

 

When I set up my brake system (Baer discs at both ends) I contacted Master Power Brakes for a master and Combination valve. They specified what I needed for my brake system and I ordered it. I installed everything per instructions using nicopp hard lines and stainless flex lines. When I first put the car on the road I was shocked to learn that the car would not stop. Not only would it not stop but no amount of pushing on the factory pedal would make a difference. One day I had had enough, I removed the combination valve and replaced it with a Kelsey Hayes adjustable proportioning valve. After a few test drives and a little adjustment the car stopped great. Before you tell me that there was some other issue like air in the system I will say that the system was bled with all the best equipment and finally using speed bleeders. There was no air in the system. My opinion is that a combination valve is for a specific design system, not for a hot rod were there are so many variables. (Tire size, brake size/ type, pedal ratio, etc.)
This is just my experience, yours might be different.

 

It is my understanding that a dual cylinder will not drain both reservoirs and you will only lose braking on the side with the leak ,I run a 69 mustang m/c no combination valve on a 56 bird

 

That has been my Southern New England rustbucket experience.

 

Diagram from the Speedway website (spelling (BREAKE) indicates that the diagram may originate from a non-English speaking country). I have used this basic setup (or slightly modified) on several vehicles. There is no fluid connection between the front and rear brake circuits - unless the master cylinder has an internal leak - the combination valve will not do anything for an internal leak.

NOTE1: RPV is Residual Pressure Valve - If you have front drum brakes, use a 10 PSI RPV.

NOTE2: Stay tuned for the "properly maintained brake system only needs a single circuit master circuit" argument. In 1964, I was a college dropout working in a gas station. The service truck was a 1956 Ford pickup - in very good condition, recently serviced my our best mechanic. Returning from a jump-start call, I stopped for a line of small kids at a crosswalk. The very next stop, the pedal went to the floor with a squirting sound. The thought that I could have mowed down a line of little tykes still haunts me to this day.

The brand new front brake line rubbed against a tire when the wheel was turned, the rim cut a notch in the rubber brake line.

Russ

 

When I built my 3W I installed a Mustang dual chamber master. Without the shuttle valve/proportioning combo and if you lose fluid from either chamber you'd be lucky to get 3 pumps of the pedal before the pedal is on the floor. It's a serious misunderstanding that having a dual chamber master will be safer than a single chamber. If all the brake lines are sealed and no issues with calipers or wheel cylinders the two systems are equality safe and functional. But unless the dual master has the above stated shuttle/proportioning valve it's not safer. To prove my point I disconnected the rear brake line on my 3W to flush the system and after 3 pumps of the pedal it was on the floor. I see many beautiful cars built with a dual master and no shuttle/proportioning valve and the owner will state how much better and safer it is than a single chamber. Not so!!!!

 

When I put power disc brakes on the front of my 57 chevy, the kit had rotors, calipers, master cylinder and power booster. There was no proportioning valve. The owner of Master Power brakes explained to me that I did not need a proportioning valve. He sold me a 'holdoff' valve and the brakes have worked well for more than 20 years. He said the holdoff valve doesnt allow any pressure to the front until the rears get (I think he said) 125 pounds of pressure. This prevents the car from swapping ends.
Dual reservoir cylinders do not connect. If you lose all fluid to the rear, only the fronts work

 

A brake differential valve is what turns on the warning light on the dashboard. It provides a ground to the electrical circuit when you lose hydraulic pressure in either the front or rear brake system. That’s the only thing it does - turns on the warning light.

 

It is my understanding that if the linkage/pedal bottoms out before the master cylinder valve finishes its stroke, the benefit of a dual master is moot. Therefore, the master cylinder valve/plunger must reach its full stroke BEFORE the pedal/linkage does. This makes sense to me and would explain why the pedal will go straight to the floor even though there may be some fluid, etc. in the brake system.

Disconnect the brake pedal linkage and with the pedal bottomed out and the linkage lined up, see if there's any more stroke in the master. If there is, you've found the reason why the pedal goes to the floor without actuating the brakes when one reservoir is low or empty. You're basically losing your mechanical ability to actuating the hydraulic system when the fluid is low due to the pedal's stroke not being adequate. When the system is full, all is well, but when it's low, the pedal doesn't have enough travel to actuate what brake fluid is left in the system. This is also why it's a good idea to bench bleed a master cylinder (you need to move the plunger through its entire stroke and many pedal assemblies won't do this, resulting in the potential of air not being fully bled out of the system).

When I installed hydraulic drum brakes in my Model A coupe, I used a Mustang dual master cylinder and setup the brake linkage so that full stroke was achieved before the pedal hit the floor. No prop valve, etc. That master has the residual valves built into the ports on the master so it's very easy to plumb.

 

Isn't it "three pedal strokes" safer?

 

If you blow a brake line or whatever you do have to pump the brakes several times to regain braking on a duel brake reservoir master cylinder. The reason for this is that either the front or rear piston in the master cylinder will have to bottom out before you will get pressure on the remaining (front or rear) system.

It’s pretty easy to test this theory if you want to make sure. Just open one of your brake bleeders, and drive it up and down your driveway to see if your brakes work. Use some common sense when you do this of course.

 

I had the same story from MPB. he also said to mount the "hold off valve" in the center of the car because if one line was shorter than the other, the shortest line would actuate that side before the longer line!!!

 

PotvinV8 is correct. When you set up the brake system you need to see that the linkage will give the master cylinder a full stroke. The differential pressure valve by itself is only to turn on the brake warning light. With a properly plumbed system, there is no place that the front and rear brakes share fluid even with a differential pressure valve. If you blow a brake hose or line , as long as the linkage will give you a full stroke you will have brakes on either the front or the rear but the pedal will be much lower. Pumping the pedal won't have much effect, the pedal will still be low, but you will have half the brake system.

 

Oh boy….a tandem master is two separate systems. That’s the point. If one springs a leak the other will still work. There is no limit to how many pumps it will work for. Will braking be diminished? Absolutely only half the brakes are working. The pressure differential valve is a light switch that’s it. If it blocked fluid flow we would have zero need for a tandem master. Many argue this but no one has shown me published service information that states otherwise.

read this.
https://www.jalopyjournal.com/forum/threads/tech-week-57-fargos-brakes-101.1229211/

 

I've always plumbed the fronts (usually discs) into the port with the largest reservoir. Only had a problem once with a leaking rear cylinder and even though pedal travel increased I was still able to safely nurse it home.

 

Just to clarify, a “hold off” valve is just another word for a metering valve. Your explanation of what it does is correct.

 

The purpose of this thread is to have a fact filled discussion explaining the rolls of the brake hydraulic system for everyone's safety- - -

In a Tandem (some say dual) Brake Master Cylinder with no leaks and adequate fluid in both reservoirs the fluid captured in the primary and secondary piston causes normal fluid movement that makes the pedal travel a short distance.
If you blow a brake wheel cylinder, hose or line , as long as the pedal linkage has been setup to give you a full stroke of the master cylinder either the primary or secondary piston that is empty will have to travel much farther for mechanical contact and movement of the pistons (not fluid movement as normal). In this case you will have either the front or the rear brakes but the pedal will be much lower because the mechanical pistons require the extra travel.

The Pressure Differential Valve portion of the Combination Valve will isolate the master cylinder on loss of one of the downstream pressures (front or rear) and keep fluid in both reservoirs which will save precious time in hard braking because the normal fluid movement in the master cylinder is working.

The paragraphs below are from https://info.mpbrakes.com/faqs

What does it mean when a Combination Valve is tripped?
The center portion of the Combination Valve is a safety feature. It is called the Pressure Differential Switch or Valve. It's purpose is to alert a driver if the brake system should lose pressure on either the front or the rear part of the system. Should a pressure loss happen, the balance will be thrown off and allow the valve to move. When it moves, or when some people call it "tripped", the internal valve moves and blocks the non-working port allowing a person to get home.


The bleeding of the brakes can also cause the valve to become tripped. To un-trip a valve, go to the opposite end of the vehicle that might be tripped to and after pumping the brakes and holding them, open a bleeder screw to try and get the center valve to move in the opposite direction. Once the line pressure becomes equal on both sides, the valve will re-center. This can take several attempts.

Here is a video

 

I spent a few minutes looking at pictures and reading the text in three 1970s factory service manuals, from GM, Ford, and Chrysler. None of them made any mention, nor were shown in any drawings (which are very detailed and clear), any mechanism to block off flow from the master cylinder to the brakes when there is a leak in one side of the system. The closest it came to that, is a mechanism that will disable the proportioning valve on front brake failure, so the rear brakes get normal pressure all the time.

youtube videos don't do much for me.

It's a neat idea, but it doesn't happen. Make sure you have your pedal set up to provide full master cylinder travel before the pedal hits the floor!

 

I lost brakes on one side of a dual MC. It didn't stop well on the rear brakes, but it did stop and didn't kill me or leave me stranded. I have lost brakes also on a single pot master cylinder and that was the ride I'll not soon forget.

So I don't know what mumbo jumbo you're talking about but in my experience I almost died when I lost brakes on a single and when I lost the fronts on a dual I made it home just fine.

 

shuttle/proportioning valve. DOES NOT .. If I remember correct Rambler was the first to have a shuttle valve to block press loss on front or rear and may not have even been a tandem MC, pre 66/67. I remember the tv commercial...

 

I have asked before for actual Factory service info…nothing. If you understand how this stuff works, IF the pressure differential valve blocked flow to the side that failed why the extra expense of a tandem master? The purpose of the tandem master was redundancy. At that time they ceased being an emergency brake and became a park brake.

 

I too had a busted front brake hose with a single pots and they are the most risky !

My point is to let people know that
the Pressure Differential Valve portion of the Combination Valve will isolate the master cylinder on loss of one of the downstream pressures (front or rear) and keep fluid in both reservoirs which will save precious time in hard braking and therefore an extra measure of brake safety.

 

My 2018 ot truck has a parking brake and its electric. One day I’ll idle down my street to see if it will activate.

 

I never tried a 70's combination valve but used plenty of 80's and 90's units.

I talked in length with Pete today at Master Power Brakes and the sentence below is true. Their unit is a copy of a 80's and 90's Chevy S10.
"The Pressure Differential Valve portion of the Combination Valve will isolate the master cylinder on loss of one of the downstream pressures (front or rear) and keep fluid in both reservoirs which will save precious time in hard braking because the normal fluid movement in the master cylinder is working. "

Yep, full master cylinder travel before the pedal hits the floor is a MUST.

 

When the Pressure Differential Valve portion of the Combination Valve isolates the master cylinder on loss of one of the downstream pressures (front or rear) it also turned on the brake warning light. These were government regulations that required this.

 

Where? Show them, along with Factory service information to back them up? If you are going to argue incorrect information you better back it up other than you tube.

I fully understand it’s pointless but the pressure differential valve is a light switch.

 

You asked so I'm telling.
I run the 1951 single pot stock brakes on my 51 Coupe.
The master is renewed. All wheel cylinders are new or overhauled. All lines are new or in like new condition. All rubber hoses are the correct ones, brand new and they fit properly. All backing plates are inspected, pads smoothed and greased for proper function. New shoes all around. Drums turned. The system had been bled and adjusted. The fluid is fresh. The brakes have been regularly adjusted, actually monthly and for a time bi weekly since they were new... The Emergency brake is functioning well.

Russ,
I'm not trying to argue the point. To each his own but I have to point this out.
That "best" mechanic was negligent maybe even close to criminal. He used the wrong part, probably to save $2.00 in 1964. That or he was in a hurry to get that service truck on the road. So he used a brake hose that was too long.
There is absolutely no excuse for a brake line to be in the rim. None! Nada! No excuse for this! He should have caught that.

 

If you run a single pot master cylinder (like I do) you need to be very diligent about maintenance and using the correct parts, etc. Someone figured out that many people are not so diligent, and by 1967 all cars in the US came with dual systems.

They both mostly work...they can both fail catastrophically if not done right...pick your poison.

 

It was NOT the wrong part, the replacement was the same length - I was there when he replaced it. The part was not "centered" between the wheel and the axle, and rubbed against the rim at full steering lock.

Human error (and brake lines cut by trash in the road) does happen. I'll take the option that gives me one more chance.

Russ