76 4x4 brake issue

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Jun 11, 2022 | 07:07 PM

Fordfarmer79

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76 4x4 brake issue

Have a 76 f250 4x4 with the two piston front disc brakes. I have the brakes bled. The pedal will hold pressure while the brakes get bled, but as soon as you go to drive it, it will loose all pressure, engine off or on, but you can pump it up and get a very solid stiff pedal with brakes. No leaks. Front and rear soft hoses look rough, but aren't keeping pressure locked on the brakes. What do you guys think? I'm thinking it's the wrong master cylinder.

Jun 11, 2022 | 07:52 PM

tbear853

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Sounds like an internal iiston leak issue in the master cylinder. The piston has sealing rings.

Jun 11, 2022 | 07:54 PM

76bigblue

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Like the person before said it sounds like a bad master cylinder. If you have power brakes a leaky master can also kill a brake booster.

Jun 11, 2022 | 07:54 PM

Fordfarmer79

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Thanks, I have one on order. Also, there is another junction block down on the frame. I assume that is a proportioning valve? It has a sensor and wire on it..

Jun 11, 2022 | 08:01 PM

76bigblue

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Yes that is the proportioning valve. When you bleed the brakes you are supposed to pull the pin on the front out to allow you to pressure bleed the front brakes.

Jun 11, 2022 | 08:05 PM

Fordfarmer79

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Quote:

Originally Posted by 76bigblue

Yes that is the proportioning valve. When you bleed the brakes you are supposed to pull the pin on the front out to allow you to pressure bleed the front brakes.

Thank you, didn't know that.