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 Post subject: Master Cylinders
PostPosted: Mon Nov 03, 2008 8:11 pm 
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Master Cylinder info by by: Dean Oshiro, Hot Rods USA


We got these with the permission of publishers long reads but well worth it. MC may look different but the purpose and inner workings are the same.

Master Cylinders: The basic design of master cylinders are single reservoirs or dual reservoirs. Before disc brakes all master cylinders had single reservoir. This was because you wanted to apply equal pressure to all 4 drum brakes. The proportioning between the front and rear brakes was regulated by the size of the wheel cylinders. Generally you ran bigger wheel cylinders in front, because it applied more pressure and if you need fine tuning you added a manual proportioning valve to the system. In the late 60's and 70's when disc brakes were being used more and more, there was a need to have a dual master cylinder, because the requirements were different when you ran disc brakes in front and drums in the rear. Remember the volume requirements of the OEM caliper? Well this high volume and more pressure required the factories to build the master cylinders so it was cheap to produce, have a large volume and met the requirements of both the disc and drum brakes. Notice the larger reservoir in the front portion of the disc/drum master cylinder and the small reservoir for the drum brakes.

ImageOEM single master cylinders are generally for drum brake applications. The earlier master cylinder had built in residual valves for the drum brake systems. This valve is needed so that the cup seals in the wheel cylinder has pressure against it preventing them from leaking. It also allows for a certain amount of pre-load on the mechanical parts. You can not use this master cylinder with built in residual valve(s) if you have disc brakes in front because of the residual valve. I have answered many questions regarding people that have installed brakes incorrectly by using a drum brake master cylinder.

If you experience a brake lock up after a few applications of the brake pedal, it is directly related to a residual valve retaining the brake fluid within the lines and not allowing the fluid to flow back to the master cylinder. The problem is either the wrong residual valve being used, a drum brake master cylinder being used on disc brake calipers, a inline residual valve plumbed in to the brake system with a built in residual valve in the master cylinder or a defective residual valve.

Most OEM tandem master cylinders will have a residual valve built in when there is a drum brake application. That is why it is important to buy the correct master cylinder based to application. Yes, you can remove the residual valve from the master cylinder, but often the reservoir is to small and it does not hold enough brake fluid for the disc brake application. So great care must be taking when using a modified master cylinder. OEM tandem master cylinders were designed to be cheap. Careful consideration should be made when selecting the master cylinder, because of the high volume of brake fluid required and pressure for the disc brake application. OEM tandem master cylinders do not produce the same volume as two side by side master cylinders. Remember the application is stacked one in front of each other so you have a limited travel and volume to work with.

Image For over 30 years race cars have used dual master cylinders, this is the use of two master cylinders that are side by side being applied at the same time. The mounting is generally done on the fire wall, but special applications have made it possible to mount these on the floor, under the dash or in a remote location. A balance bar is used to balance the force to each master cylinder. Think of a bar with a pivot point in the middle, when pressure is applied to the pivot point both ends move the same distance. Now think of the same bar with the pivot point move more to one side, when pressure is applied the shorter end will move before the long end. That is basically how the balance bar works. In a race car there is a cable connected to one end of the balance bar, this cable would go to a knob in the drivers compartment, so he can make adjustments as the condition of his brakes and road condition changes. The balance bar also eliminatesthe need for a proportional valve. On certain applications a remote reservoir(s) are used, in these applications it deletes the use of residual valves on disc brake applications. Master cylinders of this type do not have built in residual valves in them so if you have a drum brake application you will still need an inline ten pound residual valve, this is needed to retain pressure against the cups of the wheel cylinders.

There are major advantages to using dual master cylinders:
(1) Smaller diameter master cylinders can be used to increase output pressure. The design allows the application of two master cylinders being applied at the same, thereby doubling the volume output. Because of this high pressure output you will not need a vacuum booster. If you are running any type of camshaft, chances are you do not have enough vacuum to run the booster anyway. (2) The balance bar eliminates the use of a proportional valve and gives you the optional remote adjustment. (3) The remote fill applications deletes the need for residual valve normally used when the reservoirs are lower than the calipers.Image

When calculating the output pressure of each master cylinder you can not say that applied pressure is “shared” equally between the two (2) master cylinders. If the two master cylinders did not have a balance bar between them and the application of force was always equally distributed this statement would be true. The balance bar allows the applied pressure to be distributed unequally.

Example:

6:1 ratio pedal assembly
¾" master cylinders
Applied force of 100 pounds with your foot

The formula shows that this combination produces 1359 psi, however if you apply the 100 pounds of force to both of them equally it will only produce 50 percent or 679.5 psi.

What the balance bar allows you to do is apply 65% of the force to the front and 35% to the rear so the actual output pressures would be 883 & 475 psi.


This is how you are able to obtain maximum braking with the same amount of applied force. When you are using a tandem master cylinder (OEM type inline bore) the output pressure is equal in both ports and the only way to reduce the pressure to the rear braking system is through metering (distribution block, combination valve or engineering in the master cylinder) or proportional valve. This works fine when you have more than enough pressure with a power booster but when you are using a manual master cylinder this energy is “wasted”.

Image

Here is a picture of the master cylinder mounting in one of my chassis I built. We have taken the 30 year old technology of dual master cylinders and applied it to this chassis. We used two 3/4 master cylinders with an out put pressure of 1359 psi each. This special mounting bracket mounts the master cylinders one on top of the other taking only 2.5 inches of width along the frame. The balance bar system allows full adjustment of the pressure balance and the remote fill eliminates the residual valves. This space saving feature provides additional room for your exhaust system. The deletion of the power booster also eliminates the need for vacuum.


Last edited by Leatherneck on Wed Nov 05, 2008 11:02 pm, edited 3 times in total.

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PostPosted: Mon Nov 03, 2008 8:21 pm 
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Allow me to be naive but where exactly in a vehicle is that system mounted? And that is one strange lever/pedal/scythe you have to actuate it.


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PostPosted: Mon Nov 03, 2008 8:31 pm 
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Location: Mohave Valley AZ.
This info is from Brakes for your Hot rod. This brake setup probably T-Bucket. Not our setup but good info on what is available and can be adapted to our use but other methods of mounting.


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 Post subject:
PostPosted: Fri Nov 07, 2008 7:25 am 
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Quote:
You can not use this master cylinder with built in residual valve(s) if you have disc brakes in front because of the residual valve. I have answered many questions regarding people that have installed brakes incorrectly by using a drum brake master cylinder.


So this would be a problem with rear disk brakes, and one of the problems of bleeding the system as well as less than normal pedal height. All because of the wrong master cylinder.

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PostPosted: Fri Nov 07, 2008 10:07 am 
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hmmm, I'm wondering if this may be a problem my friend is having. We put disk brakes on his buggy (no front brakes) and used a "buggy style" (early VW single circuit) master cylinder. The brakes worked beautifully at first, but now, they stop well, but the pedal is rock hard.

Does the commonly sold "buggy master cylinder" have any type of residual valve built in? Could this cause a firm pedal?


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PostPosted: Fri Nov 07, 2008 10:45 am 
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fl_buggy that is a very good question maybe Dean will see this post and give us some input he was sent a link to the info that is his and asked for permission to post it up. :wink: It does bring up some interesting questions.

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PostPosted: Sat Nov 08, 2008 1:05 pm 
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Image

Quote:
Replacement Type 1 dual circuit master cylinders available now do not have any residual pressure valves fitted and the VW type screw-in units don't appear to be available any more.

Luckily, there are several specialist brake component companies that still manufacture 10 and 2 psi residual pressure valves, as pictured above, which fit anywhere in the brake lines.

http://www.shining-wit.net/tina/buggy/d ... index.html

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PostPosted: Sat Nov 08, 2008 1:16 pm 
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So this brings up another problem. If you have discs in the rear and a cutting brake. Should the pressure valves for the rear discs be behind(downstream from) the cutting brake?

For the front valves, can you mount them right on the master?

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 Post subject:
PostPosted: Sat Nov 08, 2008 1:44 pm 
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Image
It says that
Quote:
Without the Residual Valve the inertia of fluid returning to the Master Cylinder may cause a vacuum and allow air to enter the system.
:?

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PostPosted: Sat Nov 08, 2008 1:58 pm 
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I continue to have brake problems after replacing every single part on a 1972 bug. I still have no pedal pressure, I can still push the pedal to the wall. Could adding residual valves solve my problems??


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PostPosted: Sat Nov 08, 2008 2:25 pm 
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I have the bus single cylinder with the cheap Type III discs and I have a similar problem with them getting a hard peddle pressure. Though experimenting, I found mine is on the left side which is not the side I usually turn to (every one has a tendency to turn predominately to one direction or the other).. I don't think the old single piston bus unit does have a residual pressure valve in it. I am thinking that is might be in the turning brake or it could be in the less used disc on that side. I have to bleed them to see if that makes a difference. I think I read that some of the dual cylinder MS may have a built in valve but I am not sure about it.

As far as the residual pressure valve, remember it is used when the slave (wheel cylinder or caliper) cylinders sit higher than the MC (bleed back). Since, if you have only rear brakes, the line out of the MC is single then one valve should be required somewhere before the line splits. With four wheel brakes, and no turning brake, then either two or three valves would be used depending on the MC outputs are.

The problem I see is if the turning brake is above the wheel cylinders and above the MC, then the valve for the rear would be at, or between the MC and the turning brake as that is where the bleed back would most likely occur.

I have talked to some rail owners with higher dollar discs and they use the 10# valves rather than the 2# valves with seemingly no problems.

If you look at car placement, street rod diagrams are the best example, they have one for each wheel, in line. Stock production cars have them in the splitting devices by the MC. or at least did.

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PostPosted: Sat Nov 08, 2008 3:00 pm 
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http://www.thebrakeman.com/valvetechi
Lots of info here I get e-mailed the Brake Man and asked if I could post his info here, since it is his info.

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PostPosted: Sat Nov 08, 2008 3:34 pm 
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I don't know where I found this info as I saved it without a link. :roll:
Quote:
Steering brakes are often blamed when other brake problems occur. To check if the steering brake is working properly just apply a slight amount of brake with the foot pedal and then work the steering brake. If it's tight and the handle doesn't keep moving then the problem is else where. The turning brake will not move a lot of fluid so keep the brakes adjusted up-tight.

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Hot, humid air is less dense than cooler, drier air. This can allow a golf ball to fly through the air with greater ease, as there won't be as much resistance on the ball.


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