Bad MC? rear reservoir draining upon bleeding

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Master of Nothing
Dec 26, 2006
Mt. Millis
I've searched the FAQ and searched the forum threads and haven't found an exact answer.

I installed a new MC into my FJ55 which has 4 wheel discs. I "bench-bled" the new MC after it was installed.

As my helper person was pumping, the rear MC reservoir went down in fluid level by about 80% to almost dry. It took 8 pumps of the pedal to get the fluid level to stop dropping. I bled from the passenger rear caliper several times and not much improvement. When the brake pedal is released, the fluid slowly belches back into the reservoir to original fill levels.

The front reservoir doesn't drain at all and remains the same when the brakes are pumped.

Do I have a bad MC?
I'm not sure I understand...the rear fluid reservoir empties while you're bleeding the rear brakes? That's normal. The rear reservoir empties just due to pumping? and then refills itself? That sounds like air in the system.
Before I crack the rear bleeder screw, you can pump it 8 times and get most of the fluid out of the rear reservoir. I then crack the bleeder screw and it seems to be all fluid gushing out.

Basically it empties when pumping then refills itself. I've bled it at least 15 times. Maybe I need to use one of those vacuum bleeder tools. So it's just air? Seems like some of it would have worked out by now. Why would only one do it and not the other when I did essentially the exact same thing?
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That sure sounds like air. The air is compressed as you pump it up then pushes fluid back into the MC when the pedal is released. Keep bleeding, also search for brake bleeding and traped air. There are some ticks. I like to put a hose on the bleeder and run that into a jar.
You only have air in the rear circuit?

Probably so but seemed odd that since I replaced the MC and pulled the same lines off that the same would happen to both sides. Guess I got lucky on one.

I'll keep bleeding. Thanks!
Bad MC? rear reservoir draining upon bleeding

Are you sure that the bleeder is at the very top?
I had to remove a mounting bolt on a Eldo caliper and move the cyl. to get the bleeder at the very top.

Also if you are bleeding slowly air can stay on the top of a high spot in the lines.
If you think that may be the trouble, Have someone remove both bleeder, stomp the pedal to the floor and hold it down then have your helper rep[lace the bleeders.
Then bleed each caliper as a small amount of air will get in as you replace the bleeders

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