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I am getting a sinking brake pedal on my truck..when I push down on my brakes they go in..stop..sink a little.. and stop again..the brake pads are new and I have fluid in the master cyl..dont know the history on this truck..bought it used.. rear shoes are adjusted up..maybe air in line? bad master?? bad power booster???dont want to just start throwing parts at it.. thk!!
 

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Air in the lines will usually give a 'low and spongy' pedal.
As long as there are no external brake fluid leaks (reservior stays full), then the the master cylinder may have an internal leak (fluid is leaking past the rubber plunger cup when applying the pedal). This can be from worn cups or contaminated fluid (sediment allowing internal leakage past the cup).
 

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Discussion Starter #3
A couple times I have seen some fluid that looked like it leaked out the top of the master..I thought it might have been too full.. the level looks ok tho..about a half inch below bottom of the inner circle..but no leaks around the wheels
 

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That seems a little low. I fill it to the edge of the bottom circle.
 

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I think if I were you the first thing I do is wipe off the area around the master cylinder to make sure its clean and then try bleeding the brakes. Look around the master cylinder for any leakage. If you cant get the brakes to get a nice firm feel track all the brake lines looking for leaks and if you cant leaks in the lines, wheel cylinders or calipers i bet a new master cylinder is the solution.
 

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Bleeding the brakes may be a good idea, might just be air in the calipers, but the description of brakes, sinks, then brakes again, I would go with what Imperial Crown said about the fluid going past the rubber piston in the master cylinder. This description does indicate the front calipers are being filled by the master cylinder being engaged, then, as the fluid bleeds past the MC piston (sinks), then stops again, when it stops again is an indication the rear brakes are taking up the slack (rear brakes are engaged less than the front to begin with). Air could do this, fluid level not being at the underside of the lip can be from air bleeding itself out of the system slowly, so fill and monitor. If after bleeding the brakes results in the same symptoms, time for a master cylinder.
 

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Discussion Starter #7
great guys!! thanks to all.. will get my son to help me bleed them out...good father / son chore for this sunday :D
 

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Sorry to hear that, but glad it did get fixed.
 

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Glad I found this post. I'm having the same problem on my '87 Dakota. The only additional detail is the BRAKE warning light comes on. The fluid level is slightly low in the front half of the MC, but full in the back half. Sounds like I can save myself some time by only bleeding the brakes AFTER installing the new MC. :)
 

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I replaced the MC on this truck a little over 3 years ago. Shouldn't the MC last longer than that?
 

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If you don't know the history, then bleed the master cylinder first, then bleed all 4 wheels, starting with the furthest away. If the previous owner put a master cylinder on without bleeding it, that will cause problems, and just bleeding the wheels won't do it.
 

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I'm having the same problem on my '87 Dakota. The only additional detail is the BRAKE warning light comes on. The fluid level is slightly low in the front half of the MC, but full in the back half. Sounds like I can save myself some time by only bleeding the brakes AFTER installing the new MC

Nope. Waste of time. You must bench bleed the master cylinder first before bolting to the power brake booster. To do otherwise only prolongs the inevitable and creates lots of frustration. Brake warning lamp illuminating indicates significant pressure differential between the 2 hydraulic circuits in the brake system. That is indicative of trapped air, a leak, or an internal seal in the master cylinder leaking.

I replaced the MC on this truck a little over 3 years ago. Shouldn't the MC last longer than that?
Depends upon the quality of rebuild. If the replacement master cylinder was purchased at AutoCrap / AutoZone it might only last 6 months. I have experienced master cylinder failure in a short time frame with AZ rebuilds.

You mean air can still be trapped in the MC even when you bleed the wheels?
Yes. If you use the 2 person method where a helper pushes the brake pedal to actuate the master cylinder and force fluid through the system and into the wheel cylinders and calipers while another person opens and closes the bleeder valves, you are not moving a large volume of fluid due to the small diameter brake lines. You need to move a large amount of fluid quickly to carry along air bubbles to a bleeder port so they can escape. That is why a professional technician will use a power brake bleeder to force a larger amount of fluid under pressure through the system.
 

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Regardless of whether the MC is bench bled or not, the rest of the brake system still has to be bled since the lines have been disconnected from the MC for installation. So like I said, reading the original post has saved me the time of bleeding the brakes prior to just going ahead and replacing the MC because stevochar already tried that and it didn't fix the problem.
 

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You must ALWAYS bleed a master cylinder when it's been replaced. ALWAYS, first and foremost. Then you must bleed all 4 wheels in order.

You might not need to replace the master cylinder, it might just need bleeding. Try bleeding it first, then the wheels. If that fails, then replace the MC and repeat. Saves $50 if it was just air.
 

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I also had a dropping brake pedal in early December, right after our first cold snap.

Had the brakes bled and it ended up there were ICE CRYSTALS in the brake fluid. This was preventing it from flowing properly and achieving proper braking pressure

How the heck did water get in my brake lines?
 

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lalunette said:
I also had a dropping brake pedal in early December, right after our first cold snap.

Had the brakes bled and it ended up there were ICE CRYSTALS in the brake fluid. This was preventing it from flowing properly and achieving proper braking pressure

How the heck did water get in my brake lines?
Brake fluid aborbs moisture to help prevent corrosion in the brake system. Sounds like a good idea to replace all of the brake fluid with new, if they didn't already.

Bob Lincoln said:
You must ALWAYS bleed a master cylinder when it's been replaced. ALWAYS, first and foremost. Then you must bleed all 4 wheels in order.

You might not need to replace the master cylinder, it might just need bleeding. Try bleeding it first, then the wheels. If that fails, then replace the MC and repeat. Saves $50 if it was just air.
Thanks Bob. I'm pretty sure I bled the MC when I installed it in '09, but I'll give it a try. One of the other posts said to pump 70 times - I know I didn't do that.

I'm curious - where is the sensor that triggers the BRAKE warning light?
 

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I don't believe that brake fluid is DELIBERATELY hygroscopic. If it were, then there would be a maintenance interval prescribed for its replacement. It's just chance, or a negative factor, that it absorbs moisture. And it does so, very readily.
 
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