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1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks Discuss the Early Eighties Bullnose Ford Truck

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Old 03-15-2009, 10:17 PM
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Brake pressure differential switch and bleeding

Hi all,

I have a really soft brake pedal and so far I've flushed out the fluid, replaced the brake master cylinder (bench bled with a kit and rebled the brakes), adjusted the rear brake shoes and at the moment I can alteast pump the pedal a few times to build up some pressure but the truck does not give me much confidence with its stopping power.

I noticed in the Haynes manual it calls for pulling the pin out on the brake pressure differential valve before bleeding the front calipers but I can't either pull it out or push it in any with moderate force (it will jiggle a little bit so it doesn't seem "stuck"). I wonder if there's something going on with this part that is causing much of the soft pedal?

It feels like a bad brake master cylinder where you push right through the front circuit before hitting the rears. According to the picture of a similar one I found online it looks like you can take this puppy apart. I was thinking of trying that to see if some gunk was stuck in there causing my problem but wanted to check with y'all first. Thoughts?...

(I don't loose brake fluid, it doesn't leak all over anything that I can see, I was able to use a handheld vacuum pump to suck fresh fluid through each bleeder screw, etc.)

The pin is "C":

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Old 03-15-2009, 10:28 PM
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I have never had to mess with the pin, or ever had a proportioning valve give me any problems. And I have had a lot of stuff go wrong with the brakes on different trucks.

One thing to check is to make sure your bleeders on your calipers and rear wheel cylinders are at the top, higher than the brake lines where they connect. If for some reason the bleeder is low, you will never get all the air out.

And for the rear cylinders, you really have to use a lot of fluid to get all the air out, more than the master cylinder will hold, so you have to stop and refill.

Pumping the pedal to make it rise still sounds like there is too much travel in one of the brake systems, and it's usually the rear shoes. It's simple really, the shoes are too far away, so one stroke does not move the wheel cylinders enough to make contact. So you pump and pump till the shoes make contact. After that, the springs pull the wheel cylinders back in inward, and you lose it all again. Sometimes a good hint that this is the problem, is too pull the parking brake. This will take up some of the travel, and the pedal will be a little bit higher if you leave the parking brake on while you push the brake pedal.
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Old 03-16-2009, 12:01 AM
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Thanks for the quick reply and tips. I'll try the parking brake idea next.

So what may be happening is there is an issue with the rear drums or lines that causes there to never be enough pressure in that line to let the porportioning valve engage the fronts. That must be why the truck can barely stop during normal driving, I'm using all rear brakes.
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Old 03-16-2009, 01:03 AM
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`Does your vehicle have a power booster ?
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Old 03-16-2009, 09:45 AM
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Quote:
Originally Posted by sctrt View Post
`Does your vehicle have a power booster ?
Yes, there is a brake booster.
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Old 03-17-2009, 01:19 AM
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Then check your rod distance from boster to master cylinder. Have someone very gently step on brake while you watch fluid in master cylinder for bubble. Bubble should appear about 3/8 to 1/2 inch of brake peddle being depresed. Adjust booster rod accordenly. Let me know your outcome.

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Last edited by sctrt; 03-17-2009 at 01:22 AM. Reason: more info
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Old 03-17-2009, 01:19 AM
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