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Unread 04-08-2010, 04:03 PM   #1
1984Laredo
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Can't get a hard brake pedal with engine running

I am at a loss on how to get brakes on this Jeep. I just replaced the proportioning valve because the old one was leaking. I STILL cannot get a good, steady pedal.

Here's the odd part - I can get a decent-height, hard pedal as long as the engine is off. However, when it's running and the brake booster is getting vacuum, the pedal goes straight to the floor with no effort whatsoever. I understand this is the booster working, but it seems to be boosting a little too much. I have bled and bled and bled and bled and bled. No air coming out.

Everything in this system has been replaced except for the front brake pads, the metal lines, and the booster. The rear brake have been adjusted to spec. This is the second master cylinder (this one is a new one that has been bench bled 3 times so far).

The new proportioning valve came from Inline Tube and was bled per their instructions - they say DO NOT touch the pin on this valve. Rather, bleed as normal. They said I still have air, but NO AIR IS COMING OUT. So I bled as normal and followed their troubleshooting instructions. The valve is still centered, but when bleeding the fronts, the pedal will only go to a point and stop. When bleeding the rear brakes, it will go all the way to the floor. Why??

Also, the booster has started making a hissing sound after the engine is turned off, leaking down the vacuum it had. It didn't used to do this... Is this related to my issue or is this a problem at all?

What can I do to get the same hard pedal I have when the engine is not running?

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Unread 04-08-2010, 04:42 PM   #2
reef2003
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I am having the same issue, anyone got any advice....I do plan to use a Motive pressure bleeder system this weekend to see if that helps. I will let you know.
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Unread 04-08-2010, 05:02 PM   #3
Truck633
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points to a vacuum leak, with it running spray some WD 40 around the lines and fittings of your booster and listen for any engine rpm change, if you notice it, you have a vac leak, if not I would re bleed and check the master cylinder fittings.

Tom
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Unread 04-08-2010, 05:06 PM   #4
1984Laredo
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I thought of the vacuum leak and sprayed carb cleaner all around the check on the booster and at the nipple on the intake manifold. No change in RPM. I've also tightened and re-tightened everything. There are no fluid leaks anywhere.
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Unread 04-08-2010, 06:57 PM   #5
josh97
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If u start the engine and let it run for a few min and shut it off..try pulling the brake booster vaccum line from the booster and pull the valve at the same time.....It should sound like it still has vaccume..if it doesnt then the booster is bad or the valve is bad...
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Unread 04-08-2010, 08:04 PM   #6
1984Laredo
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Quote:
Originally Posted by josh97 View Post
If u start the engine and let it run for a few min and shut it off..try pulling the brake booster vaccum line from the booster and pull the valve at the same time.....It should sound like it still has vaccume..if it doesnt then the booster is bad or the valve is bad...
Not really following you... If I pull the check valve off of the booster after the engine is shut off, there should be a hissing sound from the built-up vacuum, correct? If not, the booster or check valve is bad?

Wouldn't a faulty booster cause a harder to press pedal instead of an extremely soft one like I have?
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Unread 04-08-2010, 08:38 PM   #7
brownbagg
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when my vaccum booster when bad you had to stand on the brake, super hard pedal, not a soft pedal, and everytime you hit the brake you hear air
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Unread 04-08-2010, 09:36 PM   #8
josh97
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Yeah u should hear a hissing sound...And its possible that the pedal could go to the floor...Do u still have brakes or do u loose them completly??? I was just wondering if its possible that the when the booster puts power to the rod in the master cyl it is bypassing inside the cyl??? possibly a bad master cyl seal???
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Unread 04-09-2010, 06:12 AM   #9
Mike Romain
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When you are doing the bench bleed, are you getting the plunger to stop hard 1/8" in? It took almost forever for me to get the last one I bought to bench bleed to this point, likely because I didn't have a vise to set the master cylinder 'perfectly' level in, I was just using a concrete step. The plunger would always go past the 1/8" mark. "Finally" those last little beads of air came out and the plunger came up hard when I found 'just' the right place on the concrete step.

The pedal coming up hard when you bleed kinda implies the proportioning valve is kicking in, but you had the trouble with the old valve right.
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Unread 04-09-2010, 08:18 AM   #10
1984Laredo
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I bench bled the master in a vise using the plugs and only going to 1/8" stroke (yes mine took forever too). On top of the, I took 2 old, small sections of metail brake line and screwed them into the MC. Then I attached shirt sections of clear tubing to them and ran them over into the MC, submerging them in fluid. I bench bled this way many times until I finally saw no air bubbles in the clear tubing.

Right, the pedal gets pretty hard as long as the booster is not "activated". It goes a little lower than I would like, but IIRC from my other CJ, it must be the average pedal height. With the engine not running, the pedal will go down even with the accelerator pedal. With hard pressure, it will go just a tad further. As soon as the engine is started, it goes straight to the floor without any effort to push on it at all. (Is this normal pedal height - down to the accelerator pedal?)

When it's running and goes to the floor, I barely have any brake. It will stop, but not quickly at all.
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Unread 04-09-2010, 08:47 AM   #11
Mike Romain
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If you pump the pedal, does it come up higher and harder?
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89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
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Unread 04-09-2010, 09:00 AM   #12
1984Laredo
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Pumping the pedal never changes the pedal height, both with the engine running and shut off.
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Unread 04-09-2010, 09:07 AM   #13
Mike Romain
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Quote:
Originally Posted by 1984Laredo View Post
Pumping the pedal never changes the pedal height, both with the engine running and shut off.
OK, no air then.... Air will pump up normally.

The rear shoes are up nice and snug to the drums right. No chance the star wheels are in the wrong side so they just unadjust fast or have you even driven it?

Is the tub thicker than stock or something? Is the connecting rod from the pedal to the booster adjustable and has maybe come loose at the adjuster?

I am wondering if maybe something has physically let go inside the booster.
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86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
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Unread 04-09-2010, 09:33 AM   #14
1984Laredo
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Quote:
Originally Posted by Mike Romain View Post
OK, no air then.... Air will pump up normally.

The rear shoes are up nice and snug to the drums right. No chance the star wheels are in the wrong side so they just unadjust fast or have you even driven it?

Is the tub thicker than stock or something? Is the connecting rod from the pedal to the booster adjustable and has maybe come loose at the adjuster?

I am wondering if maybe something has physically let go inside the booster.
New shoes on turned drums. All recently adjusted before I started bleeding. The adjusters are on the proper sides because I'm able to adjust the shoes moving the brake adjustment tool in the proper vertical motion like the FSM says.

The tub is stock. The booster is a stock CJ unit, so it's non-adjustable.

When looking down at the booster from above, the piston that goes in the rear when you push the pedal makes full travel into the booster, both with the engine running and shut off.
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Unread 04-09-2010, 09:52 AM   #15
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And the caliper bleeders are pointed up.
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