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Old 12-11-2011, 06:29 PM
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Valves for drum brakes and line lengths

We are building a rat rod out of spare parts. We are to the brakes, and need to know if we need residual pressure valves on drum brakes front and rear..... Also some say that the left and right lines have to be the same length....
thx for the help
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Old 12-11-2011, 06:48 PM
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Depends on where you mount the master cylinder. If its above the brake cylinders (on the firewall) NO, if its under the floor, YES.

I don't know who ?started that "lines have to be the same length" crap, but they obviously didn't understand fluid dynamics. No, they don't need to be the same length. If you look on your everyday driver they're not, so why in the blazes would someone think they have to be the same length on anything else?

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Old 12-11-2011, 07:08 PM
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Residuel pressure valves retain a minimun brake puessur to help eliminate excessive pedel travel in drum brake system. this can be done with a proportioning valve witch is what you would need.
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Old 12-11-2011, 07:14 PM
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Thx Centerline, I thought that I was being fed something???
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Old 12-11-2011, 07:16 PM
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Proportioning Valve
The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes.

The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located.

If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. If the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.
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Old 12-11-2011, 07:35 PM
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Thx Winginit.... That helps a lot...You and Centerline helped a lot.....Hope to have this project roadworthy by spring.....
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Old 12-11-2011, 09:09 PM
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Quote:
Originally Posted by winginit
Residuel pressure valves retain a minimun brake puessur to help eliminate excessive pedel travel in drum brake system. this can be done with a proportioning valve witch is what you would need.
A proportioning valve will not maintain pressure, but a residual valve will. However, there are some factory models that have a built in residual valve but they aren't all that common.

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Last edited by Centerline; 12-11-2011 at 09:19 PM.
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Old 12-11-2011, 09:17 PM
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winginit is right that a proportioning valve might be required. You can build the system without one, test it and if you get premature rear brake lockup then just install one. Cars can be different and some won't need one where some will. You can also design the system to do the same thing by installing slightly larger bore rear brake cylinders than the front which will require a little more pressure to fully engage. That's what I did on my deuce and it works very well.

When it comes to residual valves.. they aren't expensive and won't hurt a thing if you install them. Just remember to use 10 lb. for front and rear if you're running all drums.

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