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Bad Master Cylinder?


mra32

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What are the ways that you can tell if you have a bad master cylinder? I'm going cray trying to bleed this out.

 

Braking is fine, somewhat mushy, but there is plenty of power (enough to lock them up at 70+ mph) but i am able to smash the pedal to the floor if i try when i'm stopped. The rear brakes are going to get bled for the 3rd time and i'm stumped if that doesnt work.

 

Can a bad brake cable cause that feeling I described?

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Which method are you using to bleed the system? The early W-cars are very sensitive to this. There were times I'd have to bleed them multiple times before all of the air bled out.

 

The cable shouldn't make a difference. The cable isn't forcing the rear brakes to partially engage, right?

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Which method are you using to bleed the system? The early W-cars are very sensitive to this. There were times I'd have to bleed them multiple times before all of the air bled out.

 

The cable shouldn't make a difference. The cable isn't forcing the rear brakes to partially engage, right?

 

What would be the prefer method to bleed a 94 lumina with abs? my pedal is some what mushy after changing my rear calipers and abs wont engage on dry pavement when smashing the pedal. sorry if im stealing this thread but since im having a similar problem i though it would be okay to ask a similar question

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Truthfully the rear brakes are best to be bled under heavy pressure. Meaning that you perform one of the following:

 

- One person pumps the pedal a few times then holds the pedal down. Person 2 cracks open the bleeder, allowing air/fluid to escape, then closes it. Person 1 lifts up and repeats the pump-and-hold a couple more times.

 

- Buy a kit that pressurizes the master cylinder while 1 person goes to the RR caliper to bleed it, then the LR caliper.

 

I haven't tried one of those vacuum bleeder kits, that may work too.

 

All those other 1-man bleeder kits aren't so good with all of this.

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No ABS, 94+ rears, with the cables from the j/y car that i pulled the calipers from everything else touching the hyd. fluid is stock. I'm just using typical dot3 fluid.

 

to bleed, i've been getting people to pump the brake pedal once and i crack the bleeder. I'm not sure how the master cylinder is set up with split systems, but I dont see how multiple pumps changes anything. It should just be fluid transfer with no concern about pressure. Or am I missing something?

 

And every time I bleed the brakes, I go through about 20 cycles or more and the reservior has never been run dry. Sometimes i crack the bleeder only a little a little bit, but that should only make the process to go through the fluid take longer, right?

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When I drove my '94 Cutlass for a while with no pressure, it required a LOT of bleeding to finally get pressure back. Bleed it a LOT, and multiple pumps, yes.

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I'm gonna get some brand new fluid and try the whole system tonight. Any recommendations on fluid available at your local Pewp Boys store?

 

I think eventually, I'll get some ATE Super Blue (if our cars are ok with it)

 

I've thought of the multiple pump method and still cant come up with anything it actually does. When we replaced the clutch on my brothers car, my dad did the multiple pump thing and I was like WTF are you doing? Then I LOL'd at his ass when he got tired.

 

Has anyone experienced a bad master cylinder or know what to look for?

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Prestone is what I've always used / been reccommended. If you're doing the two person method, just make sure whoever is on the pedal NEVER lets off while the bleeder screw is open as air will get in the lines. Fairly obvious and I'm sure you know, but still.

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I have always pumped the pedal 3 times to ensure good pressure in the lines no matter which corner is being bled, one pump doesnt really do much when there is air in the lines, I am saying this with lots of experience in bleeding brakes, also if there is a lot of air in the system when you crack the bleeder on whatever caliper you are bleeding i like to tap towards the top of the caliper to release any extra air before the pedal is released

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Can anyone tell me WHY pumping the pedal multiple times does anything. I'm not trying to be a jerk. I just don't understand why that would do anything. My experience with the intracacies of brake systems is limited to designing, installing and maintaining a brake system with completely manual brakes using a dual master cylinder system for a split front and rear with bias controlled by a bias bar. I know full well that on that system that the only pressure normally in the lines is only a factor of the force you put on the brake pedal and thats it.

 

I dont know how an OEM setup is, but there could be components in the system where it does not hold true that pressure in the lines is always a factor of how hard you push on the pedal. There could be various check valves and orfices that i dont know about, but I would think that would only go to increase the vaguity in braking feel.

 

Even if there were check valves and orfices in the system any effect of them should be nullified once the brake pedal is depressed again.

 

And bleeding the brakes is not really a pressure dependent process, its more about fluid transfer.

 

I just wanna know WHY everyone reccommends the multiple pumps. I cant justify it in my head, but everyone says to do it. Well at least nothing will be hurt by doing it.

 

 

 

 

 

 

 

Does anyone know what to check for with a bad master cylinder?

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You don't HAVE to do multiple pumps, but the repeated pressure apply-release-apply-release tends to work any air bubbles to the top of the caliper, where the bleeder valve is. Not to mention, it assures the piston is fully seated against the pad, pads are against the rotor, and the pedal is as high as possible (therefore the piston in the master can travel further).

 

A bad master usually bleeds off pressure (aka pedal starts to drop) as you hold down the brake pedal with the engine OFF and power booster depleted.

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ok, thanks, Brian. I now know what to look for exactly. I got some Castrol GT LMA fluid and will run that through tonight if I can get a helper before I have to go to my girlfirend's house.

 

Anyone ever use that fluid before?

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Oh, I also forgot to mention that you want to avoid pushing the master cylinder piston past its normal range of travel. The person should not be pressing the pedal down to the floor under any circumstance with well-used or old vehicles. What happens is the piston, from usage, eventually wears the piston bore in its normal travel range, and there may be a "lip" between the worn area and unworn area in the bore. If you pass this lip by sending the pedal to the floor, it may ruin the piston seals. Pumping allows the person to build pressure up and get the caliper pistons into place without going too far down.

 

It's not so much a GM problem as it is with Fords and others (I've seen mechanics ruin master cylinders on Ford work vans and trucks this way), but still something to look out for.

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Also if you have a braking system with PCRVs (pressure conciuos release valves) it is harder to bleed the air from the system.

another good example is the Chevy 3500 vans use an external ABS unit underneath and the air has to push through the whole unit plus the lines.

 

A very good example would be my truck, My rear brakes are out of adjustmentso when i go to stop i have to depress the brake pedal twice to get a dcent pedal to stop,

reason being is the first pump does not pump quite enough to expand the front pistons and the rear wheel cylinders, therefore a second pump is required.

Bleeding brakes is basically the same concept :mrgreen:

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