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Unread 07-08-2013, 05:38 AM   #1
smitty010203
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1981 CJ5 
 
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Location: Raleigh, North Carolina
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Lose break pressure after cranking engine.

Changed both rear wheel cylinders and put in new master, bled everything. tried 2 masters. now I get no brake pressure after I cut the engine on. with the engine off everything feels good. Why would I get no pressure after cranking the engine?

Details:
As far as I know all stock brake system. Disc in the front Drums in rear. Power Brakes. When I got the jeep i replaced the master with a reman. a couple weeks ago I noticed brake fluid leaking around my back passenger tire. I decided to replaced the wheel cylinder, I ended up replacing both rear wheel cylinders. the adjuster piece is pretty much seized up so it never changed and I installed the shoes and everything back the way they were. I put a new reman master on also since the old one was leaking at the cap, filed the new one and bent the latch for a tighter seal. After bench bleeding the reman and the brakes after installing the wheel cylinders, my brakes seemed fine, until I cut on the engine. The pedal goes to the floor. took the reman back, got another one (bench bled it very thoroughly. still the same problem. Shouldn't be a problem with the booster i wouldnt think, and I have tried 2 different masters. What else could it be? I do get enough brake pressure to stop the brakes when I roll a few feet in the driveway.

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Unread 07-08-2013, 07:39 AM   #2
texas1al
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It sounds like a problem with the booster since all appears fine with the engine off. Unplug the vacuum line at the booster, plug or cap the line and take it for a slow drive on an empty street. With the booster unplugged, you've got the equivalent of manual brakes and while it will take more pedal pressure than you're used to to apply the brakes, if it stops ok, then you've isolated the problem to the booster.
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Unread 07-08-2013, 07:56 AM   #3
RHEEMO
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I had some issues with the brakes on my CJ acting weird having pressure on and off. I will say, the manual brakes are hard to press. I had an issue with the front calipers which caused the brakes to lose pressure at random. If you can't stop during the test drive/lose pressure on the pedal, you can push the pedal all the way to the floor and it may stop very slowly (The bottom 1/2 inch is where my brakes engaged). Be very careful driving it if you have to.
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Unread 07-08-2013, 08:03 AM   #4
smitty010203
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I figured since when i cranked the engine the pedal pressure was all gone it was the booster working so it wouldnt be that. That's what I read on some other forums. Plus I don't see why all of the sudden the brake booster would just stop working. Ill definitely give it a try though. I was thinking maybe I messed up the vacuum lines for the booster so i checked that and its still hooked up right near my carb. there are however a couple unconnected vacuum lines, its been that way though.
I know that before when I still had the tires off and I hit the brakes there was no movement at either rear brake. (engine off) is that normal? I am just almost certain I installed everything back correctly.

Thanks for the help guys.
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Unread 07-08-2013, 08:14 AM   #5
texas1al
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The symptoms you describe are a little strange. The only part of the brake system affected by a running engine is the booster, but even if it's failed, you should still have the equivalent pedal pressure of a manual system. One other possibility is trapped air in the system. It sounds like you've bled things thoroughly, but a trapped air bubble will be further compressed by the additional force of the booster with the engine running which could cause the pedal to soften. A good gravity bleed of the brakes is an easy and effective way to ensure all the air is out of the lines. With a completely bled system, you should have a very firm pedal with the engine off. I've also heard bad things about the reman'd MC's. It's possible you got two bad units in a row.
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Unread 07-08-2013, 08:34 AM   #6
smitty010203
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The pedal feels great with the engine off and I am almost certain I have bled everything thoroughly. I am going to check out the booster vacuum lines again later. I was messing around in the engine near the lines the other day and its entirely possible I pulled a hose. I have a picture of how its connected to my carb i just cant get it to load. I know that the line from the booster is still hooked up. it goes into a small block which connected to the carb, but another hose goes into that block, basically its a T. I guess I need to trace that other line again. And yea remans arent the best but really dont think both of the masters were bad.
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Unread 07-08-2013, 08:40 AM   #7
smitty010203
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This pic may be to small. you can see the small block with the booster hose connection. What is the smaller hose on the other side of the block supposed to be hooked into?
brake-boost-vacuum.png

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Unread 07-08-2013, 09:06 AM   #8
texas1al
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That open connection on the t from the intake needs to be either plugged into something or capped, otherwise you've got a huge vacuum leak and little or no vacuum to your brake booster. A lot of people like to cap that port so the booster gets a dedicated source of vacuum. Now I'm really stumped about your situation since your booster may be getting little vacuum with the engine running. That open connection was used for many things over the years and you'd have to look at a vacuum diagram for your year and engine setup. Check your underwood sticker if you still have one. It's a strong manifold vacuum feed used for emissions controls, EGR, distributor advance, etc. depending on your year.
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Unread 07-08-2013, 09:24 AM   #9
smitty010203
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well I'll be checking that out as soon as I can and I'll follow up. I appreciate all you input tex.
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Unread 07-08-2013, 09:41 AM   #10
texas1al
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The pedal will always soften a little on a power-braked equipped vehicle once vacuum hits the booster, but it definitely shouldn't go to the floor. The fact that the pedal changes at all when the engine's running means the booster is probably getting vacuum and probably working. The point of my last post is just to confirm that it's getting full vacuum off the t from the manifold for peace of mind. The pedal on the floor problem is separate and by unplugging the booster and seeing if the brakes still work will identify the booster or the MC as likely culprits. I'm betting it's the MC just by shear reputation of the reman's which means I'd be careful and slow on any test drive and be damn sure the e-brake works beforehand.
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Unread 07-08-2013, 11:16 AM   #11
smitty010203
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Ok so I just check it out at lunch. I couldn't the booster vac hose off in that time so i just clamped it with some vice grips with the engine running. doing so made a small change in the engine but more importantly the pedal started rebounding and gained some most if not all pressure back from what I could tell. Also tried clamping the smaller hose which I found T's and goes to coil and something else on the other side, didnt pay enough attention. That hose just made the engine idle lower and did not affect the brakes at all. I just dont see how the booster would all the sudden be faulty.
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Unread 07-08-2013, 11:23 AM   #12
texas1al
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The booster may be fine, it would be a real coincidence if it stopped working after your other repairs. Stupid question, but is the new MC for a power brake application? There are different MCs for power vs manual brake setups.
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Unread 07-08-2013, 11:26 AM   #13
smitty010203
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Yea it's for power brakes. Its the exact same one that I had originally put on. When i bought the jeep in Feb. I replaced the existing one with a reman, it kept leaking at the seal and was annoying me so I decided why not just swap it out for another since it was on warranty. So I did, and thats when all this started. But that's also when I changed the wheel cylinders. Now i have a 3rd MC on.
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Unread 07-09-2013, 05:26 AM   #14
smitty010203
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Just to follow up: I drove my jeep yesterday. I was managing to get enough pressure that it would drive. Its just the pedal has no pressure until its close to the floor then its snafu from there. As much as I don't want to I may end up taking it to a shop for brake service. I feel like I am missing some adjusting parts to my drum brakes. There is nothing but the star wheel bar down there that I remember. The shoes definitely need replacing and the drum is pretty beat too.
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Unread 07-09-2013, 06:31 AM   #15
RWise
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You have air in the lines somewhere,,,, or even in the MC or slave/s,,,,
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