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Unread 08-12-2013, 08:24 PM   #1
Msmall
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Cj7 - replacement master cylinder bubbles

Gents,

Replaced my master cylinder with a refurbished one from autozone. Everything seems to be functioning properly however some bubbles have me curious.

I bench bled the master and wheels per the manual. However, whenever the brake is pressed in the larger reservoir in the master cylinder bubbles a bit.

It seems to be from the initial few inches of petal.

Is this normal?

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Unread 08-12-2013, 11:08 PM   #2
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Unread 08-12-2013, 11:23 PM   #3
Skerr
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I would suggest you still have air in the master. You shouldn't be getting any bubbles, hence the reason for bench bleeding. You can still bleed your brakes as normal, but it will take longer, and you will use a bunch more fluid. But that may be more desirable than removing the master and starting over? It's all learning curve, Brother. Keep at it...
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Unread 08-12-2013, 11:47 PM   #4
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Makes sense, I don't have power brakes at the moment so I think I may just add a buster which would require bleeding again anyways. Kill 2 birds with one stone.

Am I good driving it for a week or so until I can get to it? i wouldn't even know there is an issue if I hadn't of had the master cylinder cover off.
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Unread 08-12-2013, 11:50 PM   #5
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While we're on the topic, any suggestions for a booster? What are the options that increase stopping power. I've read the master needs to be changed as well and that a booster alone doesn't actually increase stopping power, just makes the same stopping power easier to engage....
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Unread 08-12-2013, 11:57 PM   #6
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You have to be the judge as to whether you can drive it for a week. How much brake do you have? No way I would advise you on that, as I'm sure you understand. The booster can be obtained from a couple different vehicles (junkyard), or you can purchase a new/reman from the parts store. I wanna say check the GM model cars for the booster, but I'm not entirely sure about that. Someone else better say. The booster gives you "power assisted" brakes... makes it easy to push the pedal. The actual brakes/drums/rotors are what gives you the stopping power.
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Unread 08-13-2013, 01:20 AM   #7
Msmall
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Quote:
Originally Posted by Skerr View Post
You have to be the judge as to whether you can drive it for a week. How much brake do you have? No way I would advise you on that, as I'm sure you understand. The booster can be obtained from a couple different vehicles (junkyard), or you can purchase a new/reman from the parts store. I wanna say check the GM model cars for the booster, but I'm not entirely sure about that. Someone else better say. The booster gives you "power assisted" brakes... makes it easy to push the pedal. The actual brakes/drums/rotors are what gives you the stopping power.
I have a lot of brake. Maybe 1/4 inch soft in the beginning. I didn't even think of it as an issue in till I was cruising the forum. Overall you probably wouldn't know if I didn't tell you.
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Unread 08-13-2013, 08:58 AM   #8
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Just an FYI that the master cylinder used on power brake systems has a smaller bore than manual brakes. You'll want a different master cylinder if you go power.
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Unread 08-13-2013, 12:08 PM   #9
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Quote:
Originally Posted by Msmall View Post
I have a lot of brake. Maybe 1/4 inch soft in the beginning. I didn't even think of it as an issue in till I was cruising the forum. Overall you probably wouldn't know if I didn't tell you.
Then I would say Rock-N-Roll...

Quote:
Originally Posted by CSP View Post
Just an FYI that the master cylinder used on power brake systems has a smaller bore than manual brakes. You'll want a different master cylinder if you go power.
Follow CSP's advice if you choose to go power...
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Unread 08-13-2013, 08:45 PM   #10
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Thanks
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Unread 08-22-2013, 09:24 PM   #11
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I'm still having issues with this. I've bled and bled. The bubble are consistent meaning it only bubbles in one reservoir when you initially hit the brake.

Any chance I have a bad master that is always going to let air in.

I'm hesitant to pull it out on a whim.

Brake fluid starts to add up!
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Unread 08-22-2013, 09:29 PM   #12
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You are only getting air in if you are letting fluid out. If you have no leak then you have no air source. How do you know it is bubbling? Are you watching the reservoir while depressing the brake pedal with the lid off? Fluid will virtually squirt out of the top of the reservoir if you do this, making a mess! So be careful. Don't put your face directly over it.

If you have no leaks, and you are for sure getting bubbles, I would guess you didn't bleed the master correctly. Tell us your bench bleeding method.
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Unread 08-22-2013, 09:41 PM   #13
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I kiw its bubbling because I can press the brake and see it when the cover is off

I didn't have a bench so I installed the master, looped the lines from master back into master cylinder and pressed the brake until I didn't see bubbles. Then connected brake lines and bled all 4 wheels. I didn't think there would be a difference in doing this installed or on a bench. My thinking was whats the difference between the pedal doing it or my hand.

I do see a bit of brake fluid dripping from the master but I assumed that was from the bubbling leaking from the reservoir even when the cover is on.
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Unread 08-22-2013, 09:59 PM   #14
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one time I when through 2 or 3 rebuilt master cylinders on my 1984 cj with power brakes, before I got one that worked properly.

I was using advance auto, I would just take them back and they got me another one.

I bench bled the master cylinders using the inserts and short tubes provided with each kit. used the handle of my ratchet or screwdriver to pump the master cylinder, had to make many pumps until the bubbles stopped.

then installed the master cylinder and all was good,
once I got a good master cylinder.

brakes on a old jeep are marginal at best, power brakes make it much better.
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Unread 08-22-2013, 10:10 PM   #15
Msmall
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I do need to do the power brake upgrade. Part of the reason I've put the minor bubbling on the back burner.

I also want to k so what is going on with this. It seems like when the brakes are engaged and the rod is pushed into the master that it is forcing air in. As it e rod is catching air when out and then pushing air in when engaged.
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