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Discussion Starter · #1 ·
My right rear brake caliper ('89L) was freezing (which sucks since I had 2 loaded rear calipers installed last brake job) so I had to have it replaced. I also had him replace the pads all around since all were pretty low. After the install, he bled the one right rear wheel but the brake peddle never got firm again. He clamped/cutoff the replaced cylinder but still the peddle was soft. There are no signs of leaks anywhere so of course the next guess is a faulty master cylinder. (The car has 204K and the master cylinder was replaced once before at 75K.) I suggested maybe try bleeding all wheels but the mechanic was certain the work in one wheel wouldn't put air in other parts of the system. When I asked for ideas how the work done might lead to a failed master cylinder he speculated that when the calipers are compressed to load the new pads, you could push "gunk" back into the master cylinder valves, damaging them. That explanation seems to be stretching it a bit.

Does this seem plausible to others? Anyone have ideas on what else to try before plunking $200-300 for a new master cylinder.
 

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GS Hog
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MCs can be damaged during the bleeding process. Using the pedal pump method of bleeding brakes presses the master cylinder shaft further than it normally travels (as you press the pedal to the floor). This can cause old inner seals to break their seal and cause an internal leak.
 

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In other words there's no way to remove the possibility of breaking old seals in the master cyl by doing the pump bleed process right? Because i'm not aware of any other DIY process of bleeding the brake lines.
 

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msb said:
I suggested maybe try bleeding all wheels but the mechanic was certain the work in one wheel wouldn't put air in other parts of the system.
He's correct in that statement - usually if you change out a caliper on one wheel, you will only need to bleed that one wheel, and not the others. But since I would have the equipment out already, I usually would do all four again anyway.

msb said:
When I asked for ideas how the work done might lead to a failed master cylinder he speculated that when the calipers are compressed to load the new pads, you could push "gunk" back into the master cylinder valves, damaging them.
Haven't heard that about our cars, but it could be plausible. It's a known issue with later model GM vehicles that if you compress the calipers without cracking open the bleed screw, and force the fluid back, you can damage the ABS system. But that's on the GM's.

If you don't change you brake fluid religiously, then the most likely cause of your master cylinder failure is the introduction of the new fluid during the bleeding process. Usually, if your fluid is black and you haven't changed it recently, when you replace the fluid and bleed, your MC starts to leak not too soon after. This is a known issue with Honda MC's. Do a search for more info.
 

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GS Hog
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FutureX 2001 said:
In other words there's no way to remove the possibility of breaking old seals in the master cyl by doing the pump bleed process right? Because i'm not aware of any other DIY process of bleeding the brake lines.
Actually, there are other DIY options. You can still use the pump method to bleed with a reduced risk of MC failure by sticking a block of wood under the pedal to limit its travel. There are also devices on the market that will not only reduce such risk, but also make it a one-man job. Of these, some apply pressure at the master cylinder while others apply vacuum at the bleed screws.
 

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Discussion Starter · #6 ·
no master cylinder needed after all!

The mechanic called the next morning to say the brakes where much better than we both had felt the evening before so he didn't replcae the master cylinder. That seemed kind of unlikely? Maybe he just tried bleeding the other 3 wheels and that worked but didn't want to admit it since he denied it would help the day before when I suggested it. Who knows?

All I know is I'm unemployed and was not looking forward to another $300+ tacked onto the bill for the MC replacement so was happy to hear it wasn't needed.
 

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hmmm. Sorry for posting in this thread, but has anyone felt like the brakes are not that strong? I replaced all pads around, turned my rotors and installed a new master cylinder. bleed all four wheels and even though is better, it still feels very weak considering is an all wheel braking system. i'm very confused about it :confused: even my friend's 95 civic with bad pads brakes better than mine
 
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