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Old 10-18-2010, 01:20 PM
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Single Point Of Failure: Proportioning Valve

I have a 1950 F-1 Pickup with an aftermarket corvette-style disk/drum braking system. After putting several thousand miles on the truck, I somehow got air to leak into my rear brake lines (seemingly without leaking out). Now, since at least the mid-60's master cyls have been separated into separate front/rear systems so that if you have a fluid leak in one place, the other half of your braking system should still work.

...BUT...when I got a leak I lost my brakes entirely. I was fortunate to be able to drive off the road and not hit the car in front of me and not damage my truck, but this could have been disastrous!

I first thought that the master cylinder must have leaked internally for two reasons:

1. There was no visible fluid leak, and bleeding the brakes would pull an endless supply of air bubbles.

2. Even if it was a leak or if it was air in the lines, I SHOULD NOT have lost BOTH front and rear brakes.

The master cylinder is "supposed" to be the only place where you can loose all your brakes at once, so I replaced it and still didn't have any brakes, so I replaced it again (just to make sure I hadn't gotten a bad part) and still no brakes.

Next, I plugged all of the outlets from my proportioning valve and had a rock-solid brake pedal (ie: no master cyl internal leak)...but once I hook up the rear brake line (even with the front ports of the valve blocked OR properly hooked up) I loose ALL my brakes at once again.

Is it really possible that a VERY COMMON brake proportioning valve turns my brake system into a single point of failure?...or am I just missing something.

Thanks in advance!

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Old 10-18-2010, 06:41 PM
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I have not heard of one doing that, But i have heard of one sezing up and not allowing fluid to flow. They can go bad i don't think it happens very often though. So did you notice if the fluid was by-passing through the booster or not ?

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