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post #1 of (permalink) Old 04-20-2007, 03:57 PM Thread Starter
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odd brake bleeding issues

i just finished installing a 1994 YJ power brake booster and new master cylinder on my cj.

air is getting into the system somehow, but no fluid is leaking out anywhere.

here's what happens. i bleed the rear slave cylinders starting farthest from the m/c until no more air bubbles. then i do the front calipers. and i have a mushy brake pedal and can not lock the wheels up when braking.

so i go to rebleed. there's air in the lines. now, the wierd part, if i only rebleed the rear slave cylinders and then test the brakes, they work great. by great i mean i can easily lock up my 33x12.50's. but it doesnt quite feel perfect. and i know its not safe with air possibly still in my lines.

any ideas on what the culprit could be? bad calipers?
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post #2 of (permalink) Old 04-20-2007, 04:25 PM
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Did you bench bleed the MC?

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post #3 of (permalink) Old 04-20-2007, 05:12 PM
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You are describing air in the master cylinder.
It takes a BUNCH of work to get the air out of a master cylinder once it's on the vehicle...
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post #4 of (permalink) Old 04-20-2007, 06:07 PM
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It could also be a "procedure" problem.
When bleeding - "DON'T PUMP THE PEDAL."
Pumping turns the air pocket into foam, nearly impossible to get out.
You think you got it all, but in a few minutes, once the foam turns back into an air pocket - low pedal again.

Just have your helper "Push Steady," open, close, "up", then Push steady" again.

You can still do the bench bleed procedure after it's installed, it's just harder to do.

If you got the MC from AutoChina then it may be bad - I've had very poor results from them! The only thing I'll buy from them is packaged name brand chemicals - even then, they play the "bait and switch game." What they advertise isn't what they ring up - you have to fight with them every time.
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post #5 of (permalink) Old 04-20-2007, 06:37 PM
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I use a vacuum bleeder on my brakes. I would bleed the brakes and go through a bottle of fluid then give up to come back later to the same thing. It took me a couple of times before I realized what was happening. With the vacuum bleeder, I was not getting a good seal on the bleed valve. What I thought was air in the brake system was just air entering downstream of the caliber in the bleed line.

This may not be applicable to your situation.

-- Mike
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post #6 of (permalink) Old 04-20-2007, 07:51 PM
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Got two minutes.

If you didn’t bench bleed, you put air in at the master cylinder. That air has to travel the full length of the line and through a wheel cylinder to get bled out. Even though you didn’t see any air come out the bleed screw, it was still in the line. I just hadn’t made it to the bleed screw yet.

How big a hurry you in?

If you let it set overnight with the cover not tight on the master cylinder, any left there might bubble up through the reservoir.

Then gravity bleed. Simplest process there is. Just open all four bleed screws and keep adding fluid to the reservoir as needed. You can use short pieces of hose from the bleed screw into a catch can to keep from getting fluid on everything.

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