It ain the master cylinder :(

lutherfetch

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'01 Sierra 1500 Z71 ext cab, 5.3l 127k mi

I have a mushy brake pedal that goes all the way down to the stop now. The truck is still drivable... but I am afraid to tow with it. I figured it had a bad master cylinder... so I replaced that yesterday. Zero difference. None. bled the lines 3 times. There is NO air in the lines. Pulled the tires today to replace teh pads... nope... I am stil 1/4" from hitting the wear bars... got plenty of meat left on them. I am NOT leaking fluid anywhere... so I dont think I have a cut line. What am I missing? The booster? Doesnt that get HARD when it fails? Stumped.

Thanks
j
 

NickD

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Hope I am not posting this, my new master cylinder just came in, getting ready to install it. Did you pre-charge it? What about the clearance in the linkage between the cylinder and the brake pedal?

Do you have ABS, I had to power bleed mine by hot wiring the ABS pump to get a good hard pedal, but did find a leak in my old cylinder, and my pedal went down practically to the floor like yours did. Assuming if it can leak out fluid, will suck in air, will find out if this assumption is correct or not. Pedal on mine is even harder with the engine off, when its running, power booster lowers the pedal a bit.

All I know it was working well for years, then suddenly a whole bunch of problems, just out ruling a design change. Something is wrong, really don't want to spend another 700 bucks on a rebuilt ABS.

See that ABS is now standard on all new cars, because someone said its an 18% improvement in safety for people that don't know how to drive. For me, another pain in the butt.
 

lutherfetch

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good grief... havent even thought about the abs system yet. I was hoping someone would tell me to replace the left brake caliper or something. CRAP!@!! Well, guess its time to get out the book. Have never jacked with an ABS system before.

Before I go there... anybody else seen this on these GMCs/Chevys? Never had problems with this truck. Been a great ride.

j
 

NickD

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If Gus wakes up, shhh. May tell you that you need a GMtech II scanner to ignite the ABS pump, can get air pockets in there to. Was thinking about our old Bravada, least that has a frame underneath. I just securely blocked up all four wheels on my 92 DeVille, have no idea what they were thinking with the car jack for those little notches in that thin piece of metal than runs under the rocker panel. Those start to bend. But the Caddy has some very rugged rails for the suspension, so I blocked up on those.

These tinny light cars have nothing but those rail notches to jack up, and when they rust out, you have nothing. Rails on the Caddy are good, but don't like to risk it. Came in to put on some old clothes, always seem to forget that and even have more old clothes. With all four wheels off, a heck of a lot easier to bleed and I know my rear drums need some cleaning an relubing. Mostly because they are made of ferrous oxide.

My problems started with this car when the brake lines under those plastic tabs started to rust out. Should have removed those tabs a long time ago and put some paint on them, so replaced a lot of sections. Could have said, GM should have plated those lines, but that would hurt the stockholders, can't have that.
 

lutherfetch

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GM Tech tool... I hope you are not suggestign that I ***GASP*** go to a mechanic ****COUGH***** :ROFL

There has GOT to be a way to do this using two pencil erasers, some bailing wire, and a sharp stick. How convinced are you I have an ABS problem??????
 

billr

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I think it is either the ABS or air; now do you want to reconsider that "NO air" statement? Like pray it is air? When I hear brake pedal feel is "mushy" or "spongy" it is usually air in the system...
 

NickD

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All that scanner does is let you push a button to energized your ABS pump, I found the pump connector on my car, disconnected it, found the negative and positive leads to the pump and applied 12V. Can jumper it from your battery, but I used a power supply, mine draws 5 amps by the way.

What I did was crack loose the bleeder, put on a box end wrench, connected a hose to the bleeder to a container, switched on the pump, opened the bleeder, and man, did the bubbles come out. Bleeding either with my wife pumping on the brake pedal or using a vacuum got pure fluid, even after draining out a full cup. Only took a few seconds to get out pure fluid with the pump running, and after I did all four wheels, got a full pedal. But after driving it for awhile, pedal went down again. But do have a leaky MC.

When I was checking the self adjusters by backing up and constantly hitting the brake pedal, was getting clunks from the rear wheels. I put on new shoes and hardware about 8,000 miles ago, car never was driven in snow or salt. But found a lot of rust on the lower anchors. Got that cleaned up and well grease along with the caliper tabs and backing plate. Now I am trying to get that darn thing back together again with that large U spring. Been able to snap that sucker out and snap it back in, in two seconds, but that was before my hand surgery.

I may have to hire a mechanic. Did get the front shoe in, maybe I need a two day break to get the rear one in.
 

way2old

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Just for helluvit, go to gravel lot and engage the ABS. A lot of times the ABS engagement will purge air out of the motor and allow you to bleed it easily. Just a thought.
 

NickD

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Sounds a bit too violent for me, with consequential damage to the vehicle by spitting rocks all over the place. And that air has got be released somehow.

Got one rear wheel done, no problem with the LR front shoe, have plenty of strength in my left hand to pull that strong spring out and frontward while using my weak right hand to guide in the shoe. But couldn't do the rear shoe, so got my wife to place that shoe while I pulled on the spring out and rearward. Slipped right in. That enough for tonight, don't feel like tackling the other side. Tempted to just spray some WD-40 and say the heck with it.

Sure glad I cleaned the drums and hub and coated with anti-seize last time. Was able to slide those drums off with my fingers. Want to be sure the shoes will self center before changing the master cylinder and bleeding. Could center those shoes with my fingers to reinstall the drum.
 

Wild Bill

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When all else fails I gravity bleed before manually bleeding. I open the bleeder screws (one at a time farest to closest) until I get a steady drip of fluid. Then I manually bleed. (Works about 80% of the time).

Wild Bill
 

NickD

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Didn't work on my car, left it for a couple of hours and barely got a couple of ounces out with the bleeder wide open. Hot wiring the ABS pump worked, in seconds. Question is how to hot wire it. Pulling the pump relay cover off so you could manually close the contacts was out of the question, was buried. Or just get a GMtech I or II scanner. Only $4,000.00. See the tech I also requires a bunch of modules for ABS.

Now I am not saying that my ABS isn't a tad bit gummy, already 20 years old, but did run a couple of quarts of fluid through it, and that goes fast with the pump running. My fault, wasn't exactly religious about flushing it out every year.

My old company spent a lot of money on 48 V electrical systems, but that fall through, should have spent money on developing the brake system itself, but that is a real brain buster. But apparently okay with the DOT to stick with this highly corrosive system with have now. Still wonder why that can't come up with a better fluid, really hasn't changed much since 1933.
 

Gus

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Have kept cars/trucks for 10 or more years......never replaced the brake fluid, in any of the vehicles and had no problems......problems happen when the system is left open.....since the fluid is agoroscopic, and absorbs moisture, that is where you have problems...

Have seen cars come in with tin foil wrapped around the reservoir opening....saran wrap and a rubber band.....rags...you name it....people driving vehicles by continually added fluid when it gets low, instead of repairing the leak....when the pedal is released, it sucks in air and moisture.....

Better than gravity bleeding is using the brake bleed adapter that comes with vacuum pumps....much faster....
 

NickD

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Thought that word for absorbing moisture was "hygroscopic", but what do I know!



BMW is the company that came out with yearly brake fluid changes due to the hygroscopic nature of brake fluid. But never ever read that in a GM shop manual. My old black cap still has that neoprene plunger type gasket, new one is the same, but has a white cap. Think I like the black cap betterm but will stick with the new gasket.

Instead of pitching the unused cap, maybe I should sell that on ebay since so many are missing.
 

Gus

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Nick, you are correct...have no idea where in the world I got that one :eek:.....my 7&7's don't start until 8:00 PM...
 

NickD

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Is fairly close to the word, agoraphobia that is a panic or anxiety disorder, certainly fits this thread. Agoroscopic is a cool word, think I will start using it.
 
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