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Technical Forum The main forum for jeep related discussions. Mechanically Inept... |
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#1
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Bad master cylinder or vacuum booster?
Lately, I've noticed that it takes much more effort to stop my Jeep. Yes, I know bigger tires will do this and yes, this was happening before I installed the S-10 calipers. I thought maybe it was just in my head so I had a friend drive it. He agrees that there is a problem. I then drove his and it stops much easier even with his 37s. The brakes just don't feel very powerful. I can't lock them up at all no matter how hard I stand on them. My friend can lock his up easily. We re-bled all 4 corners a couple times and it still is no better. We made sure the rears were properly adjusted and still no better.
What we fdid find is that the area on the front side of the vacuum booster (where the MC bolts to it) has fluid on it. Not enough that it is dripping, but there is definately fluid there. We pulled the vacuum line to the booster and we could hear the air going in (out?) so it is pulling a vacuum. So, is it the MC or the vacuum booster or (dread) both? If it is the VB or MC, are there a better ones out there other than Chrysler that are direct bolt ins? Why put back stock parts when better ones exist. My future plans call for rear disks and WJ front calipers if that helps. Chris |
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#4
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Typically, if the pedal goes to the floor, either quickly or fades down, or if you have to pump the pedal, it's the master cylinder. If it takes more pressure than usual to make the pedal push, it's the vacuum booster.
Make sure the booster is getting plenty of vacuum. Seeing the diaphragm move isn't an indicator, as it has no load on it. Try pushing against the diaphragm when it's moving; might find a leak or hole in the diaphragm.
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#5
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Blaine - Thanks, please do elaborate. Since I am replacing, I might as well upgrade and I'm looking for a solution.
donjr5 - The pedal feels mushy and even with me standing on the brakes, I can't get it to lock up. If I pump, it gets better The theory of my friend (Joe West - for those that know him) is that the MC has a bad rear seal and that leaked back in to the VB and the brake fluid is ruining the diaphram in the VB. Sound right? Chris |
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Given my experience with replacing the MC in my Suburban, I would have to agree that yours is probably the culprit. I had a mushy pedal which out of nowhere started going to the floor and the brakes were not very effective. Luckily, I have trailer brakes or the ride down the Cajon pass after that wheeling trip would have been quite interesting
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Any better MCs out there or am I stuck with stock? Chris |
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#9
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Then they shipped me a 9" booster to try. Still not as good as stock. I have one of the first 10" boosters they prototyped. As a matter of fact, it is the first one. The more I began to play with this stuff and figure some things out, the more I began to realize about the direction we were headed. At the same time I was fiddling with this stuff the MP was shipping me for R&D, I also had a rig with all stock brakes and a huge booster with a corvette master to compare against. What I determined was that high line pressures only drastically shortened the life of your components, mainly pads. I had enough line pressure to fold over the leading ears of the pads on the fronts and distort the backing plate and shoe pivots in the rear. I am talking total destruction of the rears. I then went to a custom set of rear shoes and had them reinforced at the pivots. New PF in the front and just lived with the short lifespan of stuff. Several things became apparent. Increasing line pressure is not the total answer. The drums are too small and are not robust enough for bigger tires. The rotors are way too small and the swept area on the pads is insufficient. What is needed for anything in the 35 and up tire size is a bigger set of components at the axles. I have done that on several and it is not an easy fix. Time consuming, expensive, and a total pain in the butt. If you want line pressure, toss in hydroboost. Easily doubles your line pressure. Be prepared for failure of stock components. In your case with the DEO stuff, I would start with stock parts. Get a new booster and master and plan to get a combination block from a ZJ. It's set up for disc and disc. I have run line pressures on Kat's with the stock booster and the ZJ master and she easily generates over 1300 psi. More than sufficient for 35" tires and good components. I am assuming you have or will have rear discs.
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I am Savvy. |
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Thanks for the info. Chris |
#11
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Diagnosing a brake booster is a straight forward procedure. With the engine off, pump the brake pedal four or five times to bleed the vacuum out of the booster. Then with foot pressure on the brake pedal, start the engine. If the pedal drops, the booster is good.
If you haven't done this, try gavity bleeding your brakes. Sometimes having someone pump the pedal to bleed brakes agrivates the problem. To gravity bleed brakes, remove the master cylinder cap and make sure its full. Then open the bleeder on the right rear and let the fluid drip. After the rr move the the left rear and then the rf followed by the lf. Good luck
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#12
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Mini tip for brakes and bleeding
Wrap teflon pipe tape around you bleeder nipple threads; this acomplishes two things; Keeps the grime out so that they don't sieze and helps keep the fluid from leaking out around the threads so the bleeding operation is much cleaner.
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2001 Sport, D44, NV3550 Rancho R/C, 8274, 32X11.50 KM's on Canyons. |
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Chris |
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