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Old 08-27-2010, 19:53   #1 (permalink)
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brake booster or master cylinder bad?

Hey guys,
How does one tell if either the brake booster or master cylinder is bad?

I'm sure something is wrong, I just can't tell what. When in park, I hit the brake pedal a couple times, rpm's don't move. When driving, if I hit the brake pedal really quickly a number of times and then hold it, no matter how hard I push the pedal, the brakes won't hold or stop the car.
Does that make sense? I'm not sure what the symptoms mean?
Any help is greatly appreciated.
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Old 08-27-2010, 21:38   #2 (permalink)
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Re: brake booster or master cylinder bad?

With the engine off, pump the pedal until there's a high hard pedal. The hold the brake pedal down and start the engine. The pedal should drop slightly. That would let you know the booster is good and the hard pedal with the engine off tells that the master cylinder is good. Maybe yo7u have brake hoses plugged or wheel cylinders stuck. Make sure the basic brake system is good and all is working.
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Old 08-28-2010, 15:30   #3 (permalink)
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Re: brake booster or master cylinder bad?

Thanks for the advice.
I'm working on getting the belt tensioner replaced as I believe its gone bad and needs replacing. I've gotten the alternator off so far. Fun stuff.
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Old 09-18-2010, 13:50   #4 (permalink)
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Re: brake booster or master cylinder bad?

Ok, I got, the belt tensioner, idler pulley and new belt put on. No more squeeks from under the hood. The belt tensioner was the problem, it was tweaked outwards just enough to cut the belt. Anyway, all is fine now.

I check what you're talking about. The pedal does what its supposed to with engine off and then on. But still, when I go to brake suddenly, and push hard, it feels like something is failing...even if I don't pump the brakes like I mentioned before and just go hit the pedal hard, they don't want to stop and it can be scary. I bought a new remanufactered master cly and will replace I guess. I don't know what else it could be or how to diagnose anything else.
thanks
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Old 09-18-2010, 14:32   #5 (permalink)
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Re: brake booster or master cylinder bad?

You may have only the front brakes doing all the work. Find a gravel area and try locking the brakes. You need the fuse removed from the ABS to try this. If they don't all lock up evenly, you'll know where the problem lies.
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Old 09-18-2010, 14:48   #6 (permalink)
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Re: brake booster or master cylinder bad?

I don't believe the car has ABS...its a 1999 GS model Sable 3.0L 2V...but the gravel idea sounds good...but I have to find a patch, hmmm.

You think possibly, the rear drum brakes are not functioning? I changed the shoes out not too long ago, perhaps I did something wrong...adjusted them too tight??
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Old 09-18-2010, 16:48   #7 (permalink)
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Re: brake booster or master cylinder bad?

What happens on the drum brakes is the wheel cylinders will rust inside and they won't move the shoes to work. Almost all cars in 1999 had ABS. You can also raise the rear, take the drums off and hav someone push the brake pedal and see if the shoes all move out.
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Old 09-18-2010, 18:06   #8 (permalink)
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Re: brake booster or master cylinder bad?

Im know expert but with engine running step on brake pedal if its hardfrom the top down or does not move then vac bosster is bad if easy to push then vac is working but master cylinder is by-passing fluid inside which means bad master cylinder. Or you could havea bad wheel cylinder that is not allowing the system to build pressure and work properly hope this helps. I konw rhe feeling of steping on the pedal and no brakes it realy sucks when you have a loaded trailer behind you.
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Old 09-19-2010, 03:23   #9 (permalink)
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Re: brake booster or master cylinder bad?

Ok, all of it is useful advice. Before i replace the mster cyl, Ill check brakes and diagnose further based on what you all have said.
thanks...and ill report back. There's another issue with the car, but one project/problem at a time (there's a steering wheel shimmy when driving, I suspect bad axles). Anyway, I'll get back to you all.
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Old 09-26-2010, 07:48   #10 (permalink)
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Re: brake booster or master cylinder bad?

I had a similar problem with my '99 Taurus SE non-ABS, last year. Both rear brake cylinders in the drums were toast. Pads and the like were in great shape otherwise, but the cylinders required replacement along with the master cylinder which I had done at a shop. One of the springs in the drum were also shot, so the shoes would drag at times. It was more an annoyance than anything as I didn't notice too much decrease in braking performance, as the rears are more for stability rather than stopping force. I have a thread with some photos floating around...somewhere...
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