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Brake Problem need help!

904 Views 10 Replies 5 Participants Last post by  TiminMb
Hey guys, I've been having this problem off and on. When I'm out wheelin just crawling around I lose my brakes on inclines. They just sink right to the floor boards. A couple pumps and they are back to normal. On the highway and city they work fine, I've blead the lines a ton of times and the M/C as well with the 2 tubes going back into the resivior. Any Ideas?
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I had a customer with a complaint sort of like your's. It was an early 90's Cadillac, and she was having brake issues. Turned out to be a leak in the vacuum booster, right around the seal where the booster went through the firewall.

Pumping them up would bring the brakes back. There was also a bad proportioning valve on the vehicle, which I believe contributed to the failure of the booster.

Those would be the next places to look. I'd suspect the proportioning valve first. If none of those work, suspect the master cylinder.

Just for grins, try bleeding the brakes again, but do a "gravity bleed". All this involves is cracking the bleed screws open and waiting a few minutes for the fluid to run clear. You'll have to maintain the level in the resevior, but this method works really well if you're bleeding them alone. Only crack one corner at a time.

My '69 Chevy pickup does the same thing, but in this case it's the self-adjuster in the front drums. The left side doesn't work well, and so sometimes I get some really exciting moments. Since you're in a CJ-7, this wouldn't really be an issue for you. Rear drums work a little differently.

Good luck with that!


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GP'n, not sure how to put the edited replies in, but I'll try to explain as best as I understand it (fully admitting that I'm always learning).

If I understand it correctly, the vacuum booster assists the manual function of the master cylinder by applying vacuum to a diagphram that in turn creates a pressure differential between a system of chambers, which assists pedal movement to apply pressure to the M/C. When the booster fails or vacuum is not present, the brake pedal can travel to the floor, or it can become very stiff, depending on the style of master cylinder and booster used. Most M/C systems have an override that will allow the pedal to be pushed (hard) to the floor and stop the car. This was the exact experience that happened to the Cadillac. This particular car also has seperate proportioning valves to control the rear brakes. One of them had failed, and was spewing brake fluid. My guess is that the system went low on fluid, the customer panic braked one time too many, and that led to the physical failure of the booster (there was an actual tear in the seal where the brake pedal pushrod goes through the firewall into the booster).

Most Jeeps I've seen use a vacuum style booster, although according to my brake book hydraulic boosters were very widely used on GM cars in the 80's. I do not know what style he has. My CJ7 had vacuum.

I don't know why the front drum system is different. I was probably talking through my a$$.
Happens occasionally. It could be in my case that the truck was converted from a manual system to a vacuum assist system, but the drum brakes were never designed for that. My handy Haynes Brake Manual says that drum brake systems do not require the same amount of pedal effort that disc brake systems do because of the "self-energizing action inherent to the design of drum brakes", and therefore power assist was not really needed until the advent of front disc brake systems. Perhaps the power assist is overriding a loose adjuster in the drum, causing the brake to occasionally lose tension. If the wheel cylinder had failed, there would just be no braking on that corner. It shouldn't affect pedal travel all that much, or else there would be big puddles of brake fluid under the wheel.

Hope I addressed your concerns
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