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Decided this would be a good day to flush old fluid and replace with DOT 4 synthetic. Not my first brake job, by a long shot... but anyway, started with removing some old fluid from MC reservoir then refilling with new. Next, began bleeding brakes at right rear and spent a lot of time there, pumping brakes and draining old fluid, until new clean fluid appeared, and then until all air was removed. Left rear went faster, but... when I started on right front, things just didn't feel right anymore... pedal would go way down and not seeing much fluid pressure at any of the bleeder valves—some, but not much. Never felt like I could get any of the four bled properly after that. (Kept reservoir topped off entire time and never let it get very low.)

 

Been at this a few hours now and don't feel like I'm getting any closer. Not seeing any fluid leaks anywhere. Does this sound like a bad master cylinder possibly? Not sure how old the unit is—looks to be AISIN. Is it possible for an old MC to go bad in middle of job, due to lots of heavy pumping?

 

Appreciate any ideas 🙂

 

IMG_4552.jpeg.b9f1a97cae2e051f695f0e2e74352aa7.jpeg

Edited by Ctgriffi
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sure could happen.  probably a $10 master. just replace

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Doubtful the master went bad from that. The masters for our heavy duty trucks start at about 60 by the way. You don't mention if you had someone working the brakes for you. It is a must unless you have a tube from the bleed valve in a container of brake fluid so it can't suck air. If you do decide to buy a new one save your cap. Many of the replacements don't include the cap and sensor. Also check your flexible brake lines. They might not show fluid leaks but when the bleed valve is shut and pedal is released they can still suck air. Makes bleeding a never-ending situation.

Linda S

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Curious situation. I recently replaced a rear axle seal and bleed my brakes. I used this: 

 

https://www.oreillyauto.com/detail/c/performance-tool/tools---equipment/mechanics-tools/brake-tools/brake-bleeder-tools/058eaccfa9b5/performance-tool-brake-bleeder-kit/pfm0/w80625/v/a/8163/automotive-truck-1987-toyota-pickup?pos=0

 

Worked and was easy. I did notice the front right then left calipers being easier to pump the fluid through than the rear had been.  My front left caliper had the most air bubbles and get rid of. The whole job using the bleeder kit took about 20 minutes. I did not remove wheels nor jack the vehicle up. Just turned the wheel on the front to access the bleeder valves; could reach the rear without a problem. 

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removing fluid fro an old mastercylinder can dislodge crud that can then foul the seals. best to never put anything but fluid in a master cylinder

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New MC seems to have done the trick. I haven't dissected the old but it was clearly leaking a bit from the main rear seal, all down the front of the brake booster (broken/bad gasket between the two as well).

 

I went with the Cardone 13-2530 which was delivered in two days for about $55; seems like a decent part and came with new reservoir, cap and float switch (I did paint the cylinder to deter rusting). I did the bench-bleed in my vise before installing and then got all the lines bled pretty easily after that. Pedal feels good and solid, and all fluid is clean and clear. Back in business.

 

 

new-cardone-mc.jpeg.0cf0d3bf880ab3b2a8aae7f164d2ae83.jpeg

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  • 11 months later...
On 8/8/2021 at 9:17 AM, extech said:

removing fluid fro an old mastercylinder can dislodge crud that can then foul the seals. best to never put anything but fluid in a master cylinder

Man you are everywhere 

 

having this problem and seeing this 

 

gonna go get a new master

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