Armed Polite Society
Main Forums => The Roundtable => Topic started by: zxcvbob on January 02, 2019, 01:01:43 PM
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I think since the late 1960's, all master cylinders have 2 pistons and either the front and rear brakes are on different hydraulic circuits, or they split them diagonally.
I haven't driven my truck in a few months. Started it up last week and the brake pedal went all the way to the floor. "Sorta like stepping on a plum." I checked the brake fluid level and the reservoir was over half full. I pumped the brakes a couple of times and then quickly looked under the truck for leaks. Sure enough I see brake fluid leaking right about where the firewall is, like there's a burst or cut brake line. But the truck has no brakes at all. The brakes were great the last time I drove it.
Bad master cylinder, right? Although if the rear seal on the MC was out, I thought I would still have brakes from the front piston.
Going to have it towed to a repair shop, just trying to make sense of it.
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"Sorta like stepping on a plum."
"Well, from there on down it just wasn't real pretty,
It was hairpin county and switchback city.
One of 'em looked like a can full of worms.
Another one looked like malaria germs!"
Yeah, likely the master cylinder puked a seal. If it's coming out at the firewall it's probably taken the brake booster with it. Not a big deal and only modestly spendy unless it's one of GM's ridiculously expensive HydroBoost units.
Front/rear splits are mainly to give redundancy in the case of a failed brake line, not an internal failure of the MC. When that goes, all bets are off.
Brad
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Master cylinder failure for sure. It is leaking out the back of the master cylinder past the piston seals.
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Water in the fluid (last summer was wet and humid) and subzero weather. Brake fluid loves water.
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Definitely sounds like a master cylinder. If it's just one leaking line, you would have at least some brake pressure, albeit not necessarily enough to drive safely.
Though not a licensed mechanic, I have worked on hundreds of cars over the years, and I have to warn you that tearing into the brake system can have a domino effect, especially if it's an older car with corroded lines.
Case in point, my 2010 Ram pickup truck. A couple of years ago, one of the soft lines in the front had an internal collapse that was causing the wheel to lock up. Replacing a flex line is usually not a big deal, but the collapse caused overpressurization in the ABS control module, so that started leaking. Being a work truck that just needs to, well, work, I didn't spend the several hundred dollars for a new module. I bypassed it using about 20 bucks worth of parts. Even had I replaced the module, just pulling the lines off of it caused ruptures in a couple of the solid lines. To make a long story short, I thought I was replacing just one soft line, and ended up doing nearly the entire truck.
I hope this does not happen to you. Good luck.
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Even had I replaced the module, just pulling the lines off of it caused ruptures in a couple of the solid lines. To make a long story short, I thought I was replacing just one soft line, and ended up doing nearly the entire truck.
Every time I see a story like this, or watch a South Main Auto or Eric the Car Guy video, I thank my lucky stars I live in a semi-arid region. Chassis and brake line rust just aren't a thing here. In my 51 years the only brake lines I've ever had to replace, or ever even heard of being replaced, were from impact damage or dry/age-rotted flex hoses.
Brad
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Just in case anyone was curious to see the ABS module bypass I described. Being just a work truck, I had no need to make it pretty. So I just arranged the new lines like a rollercoaster track to take up the slack. The T connector sends one line to each front wheel, and the other one sends fluid to the rear.
This has worked flawlessly for two years, even with a snow plow and salt spreader on the truck. Interestingly, the truck now stops better than it ever did before. Of course this means I have no ABS, but 1) I actually know how to drive, so I don't need it and 2) Plowing snow is substantially easier when you don't have a computer fighting your foot every time your truck's wheels slide.
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I just replaced the master cylinder in the freakin' rain. And the new one behaves just like the old one; except I can't tell if there's brake fluid running out the bottom because of the previously mentioned rain. So apparently I have 2 broken brake lines, that broke (or were cut) at the same time. I doubt they were cut because it's not easy to get to them where the leak is, and there are other places were the lines actually are accessible.
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Water in the fluid (last summer was wet and humid) and subzero weather. Brake fluid loves water.
This is why I've been using silicone brake fluid for the past forty years or more.
Caveat: Some (all?) anti-lock brake systems don't tolerate silicone brake fluid.
I just replaced the master cylinder in the freakin' rain. And the new one behaves just like the old one; except I can't tell if there's brake fluid running out the bottom because of the previously mentioned rain. So apparently I have 2 broken brake lines, that broke (or were cut) at the same time. I doubt they were cut because it's not easy to get to them where the leak is, and there are other places were the lines actually are accessible.
This happened to me two years ago, on a Jeep Cherokee that I hadn't gotten around to flushing and replacing with silicone brake fluid. Moisture in the brake fluid collected at a low point, where both lines come off the master cylinder and distribution block/proportioning valve. Lines looked good on the outside but were corroded on the inside. Panic stop situation blew out BOTH lines.
Note: If you lose one line, you'll still have brakes on the other circuit but the peddle WILL be much lower, and feel mushier, than when everything is right.
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From my one bad brake experience: I blew a front brake line several years back in a '98 pontiac. The brakes failed completely- pedal to the floor, no stopping at all. replaced brake line, bled/refilled brake system=good as new.
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I have had two experiences aside from the one when I blew both front and rear brake lines simultaneously. The other two incidents involved a 1988 Jeep Cherokee, and a 1988 Jeep Comanche. In each I blew the hard line leading to the rear axle. The peddle became mushier and went down farther before the brakes engaged, but I still had front brakes and the vehicles were still driveable (albeit not legally).
The '88 Cherokee was fun. The break in the line was approximately in line with the B pillar. I cut the line about six inches forward of the break, replaced the line from there to the back, and then had a friend come over to help me bleed the system. We bled it, got a good, solid peddle, and then he stomped on it as a final test ... and blew out the line six inches upstream of where I had spliced it.
So I replaced the rest of the line, from the splice all the way forward to the proportioning valve.
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I'm assuming now that 2 lines blew (or burst from freezing) I should probably get *all* the lines replaced if those are corroded. ??? :'( I will tow it to a shop that I trust and see what they say.
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From my one bad brake experience: I blew a front brake line several years back in a '98 pontiac. The brakes failed completely- pedal to the floor, no stopping at all. replaced brake line, bled/refilled brake system=good as new.
I was driving a 9th gen Pontiac Bonneville (forget the exact year but similar vintage to yours) and had a brake line burst at a stoplight. Pedal to the floor but I was able to pump the brakes to come to an eventual stop then limp into a parking lot. Just like you I replaced the line and was good to go.
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Yeah, lines + wheel cylinders (if drum brakes) / etc... pretty much assume the entire brake hydraulic system needs replacement, just to be safe.
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I just replaced the master cylinder in the freakin' rain. And the new one behaves just like the old one; except I can't tell if there's brake fluid running out the bottom because of the previously mentioned rain. So apparently I have 2 broken brake lines, that broke (or were cut) at the same time. I doubt they were cut because it's not easy to get to them where the leak is, and there are other places were the lines actually are accessible.
Did you bleed the lines after replacement? Air getting into the lines when you replace the master is common. Without a proper bleed problems can persist.
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Did you bleed the lines after replacement? Air getting into the lines when you replace the master is common. Without a proper bleed problems can persist.
No, I did not bleed the lines yet, it was raining. I did bench-bleed the master cylinder before I installed it. I expected a little air in the lines after that, and the next step was to totally flush the lines. But i got no brakes at all.
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No, I did not bleed the lines yet, it was raining. I did bench-bleed the master cylinder before I installed it. I expected a little air in the lines after that, and the next step was to totally flush the lines. But i got no brakes at all.
If you got enough air in there, you won't have any brakes until you bleed.