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Unread 02-12-2010, 10:34 AM   #16
86cj74.2L
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My service manual for my 94 Dodge Ram tells me specifically not to pressure pump the brakes while air is in the system because it will turn 1 or 2 air bubbles into 1000 small air bubbles and get distributed through out the entire system.

It says to open the bleeder before pushing the brake pedal. I've had far more success doing it this way then the old way.

After bleeding and then driving a little, I'll go back and just burp (just open it and let some flow out, say 3 to 5 seconds) each wheel and sometimes I'll get a little more air out of at least 1 wheel.

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Unread 02-12-2010, 10:43 AM   #17
JeepHammer
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Sound like you might have a bad/stuck Combo valve ('Proportioning' valve).
VERY common in vehicles this old.

Rust/corrosion/moisture collects in the valve, and the piston/valves don't move correctly, trapping air in the system you simply WILL NOT get out.

The function of the 'Combo Valve' is to,

1. Delay the front discs, which will apply first.
It will delay the front brakes slightly so the rear drum brakes that are slower to respond are applied first.
This helps you brake in a straight line instead of sliding the front wheels before the rears apply.

2. Cut off leaks to any given wheel.
When the valve 'Senses' a 'Leak', that wheel is cut off by pressure differential and you are allowed to stop without loosing all your brake fluid and having NO brakes.

3. Move a central spool valve that activates a switch that tells you your brakes are not working correctly.
-----------------------------------------------------

You *CAN* remove the piston and clean it, but that might not take care of the springs, ect. in the valve that could be malfunctioning.

With the system closed, the piston didn't move.
Once you tried to bleed the brakes, you forced air in the system and that allowed the piston/valves to move,
And now there is air trapped in/under the valve you can't get out because the piston is working against you.

I have disassembled a few and tried to clean them,
Results are about 50/50 on getting them to work correctly again.

The reason you are able to get 'Hard' pedal is the valve is moving shut when you apply pressure.
The valve is seeing 'LEAK' where the air is trapped, and moving to close off that 'Leak' so you can stop the vehicle,
When you release the pedal, the air is reintroduced into the system, and the pedal goes soggy again.

These things are VERY aggravating to get bled once they malfunction, and usually the best course of action is to replace them.
--------------------------------------

Sometimes, if you go SLOW with the pedal, down stroke with wheel cylinder open, then SLOW upstroke with the wheel cylinder closed,
You can push the air out without making it a 'Foam', but that doesn't always work either.

The idea is to push the air past the valve without making it activate fully, and that will keep the line flowing and not get it cut off from the rest of the system.
------------------------------
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Unread 02-12-2010, 02:21 PM   #18
Schne
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Would a power bleeding system work in this situation? One that pressurizes at the master cylinder and forces the fluid through?
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Unread 02-12-2010, 02:34 PM   #19
Mike Romain
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Originally Posted by Schne View Post
Would a power bleeding system work in this situation? One that pressurizes at the master cylinder and forces the fluid through?
Lots of times a plain old gravity bleed will do the trick. Open all 4 bleeders and just keep the reservoirs full for a bottle full or so.
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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.
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Unread 02-12-2010, 04:24 PM   #20
Schne
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Cool, I will give that a try after I change out my hoses. Thanks for all the help
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Unread 02-17-2010, 12:37 PM   #21
spiatt
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So what was the outcome? Did the gravity bleed finally fix the problem? Just wondering because I'm fixing to take my 79 CJ to the shop to have the brakes bleeded after I replace the MC. Hmmm I worried also because I did buy a Napa rebuild, I hope I don't have problems like some have had.
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Unread 02-17-2010, 01:29 PM   #22
Mike Romain
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Quote:
Originally Posted by spiatt View Post
So what was the outcome? Did the gravity bleed finally fix the problem? Just wondering because I'm fixing to take my 79 CJ to the shop to have the brakes bleeded after I replace the MC. Hmmm I worried also because I did buy a Napa rebuild, I hope I don't have problems like some have had.
I just took mine for a short run after replacing yet another NAPA MC and this one seems to work.

I 'didn't bleed anything though.... I unbolted the MC from the booster first and tipped the lines up when I took them off, then bench bled the MC well and put it on while it was dripping into the tipped up lines, then I mounted it on the booster again. I also had to change an axle U-joint so had the front tire already off to simply push the caliper cylinder back in. i then removed a back tire and pushed that cylinder piston back in.

By doing this, I pushed any air bubbles that were right at the MC backward through the system into the reservoir. I ended up with a nice tight pedal anyway.
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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 02-17-2010, 02:17 PM   #23
spiatt
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Ok so I have another question. Since there seems to be problems with the rebuild MCs is it better to buy the rebuild kit and just fix the existing one? My old one is leaking into the power tran but no where else. I have a mechanic who is coming this evening to remove/replace the MC for me (I don't have air tools) and I have already bench bled the rebuild. My fear is its going to leak so I'm really keeping my fingers crossed :-) He's also gonna bleed it for me.
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Unread 02-17-2010, 02:29 PM   #24
Mike Romain
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Originally Posted by spiatt View Post
Ok so I have another question. Since there seems to be problems with the rebuild MCs is it better to buy the rebuild kit and just fix the existing one? My old one is leaking into the power tran but no where else. I have a mechanic who is coming this evening to remove/replace the MC for me (I don't have air tools) and I have already bench bled the rebuild. My fear is its going to leak so I'm really keeping my fingers crossed :-) He's also gonna bleed it for me.
The kits aren't cheap and you need special honing tools and you have no warranty if you screw up....

The problem is the bore has to be honed larger and they don't sell oversized seals.
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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
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
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Unread 02-17-2010, 10:03 PM   #25
Schne
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Well I replaced the two front hoses. I then gravity bleed all four wheels, ran about a half a quart of fluid through the system. And still have the same problem, can pump the brakes up to work, then after you let off the pedle for a few seconds the presure is gone. WHAT IS THE PROBLEM, I am so lost...........
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Unread 02-18-2010, 06:02 AM   #26
Mike Romain
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Quote:
Originally Posted by Schne View Post
Well I replaced the two front hoses. I then gravity bleed all four wheels, ran about a half a quart of fluid through the system. And still have the same problem, can pump the brakes up to work, then after you let off the pedle for a few seconds the presure is gone. WHAT IS THE PROBLEM, I am so lost...........
That indicates you still have air in the system.

The 'new' Napa MC I just put on was a bit of a bear to bench bleed, it took a 'long' time to the point I was worried it might not pump up. Mine needed short 3/4" shots on the cylinder only and a whole pile of them before it came up tight so the cylinder 'finally' would then only go in the 1/8" called for, no more.

If you didn't get the MC cylinder to come up tight like that, then there is still air in it.

Oh, did you manually adjust up the rear wheel cylinders so the back wheel has friction grab from the brake shoes dragging on it? If you don't do this, then the pedal will need to be pumped to fill up the piston in the cylinders, then once let off, the cylinders relax and you have to pump it up again.

They are self adjusting when you hit the brake pedal in reverse, but they have to be manually set up the first time.

Same deal for the emergency brake cable, you want the rear cylinders manually adjusted first, then you still want a little slack in the e-brake cable. If the e-brake cable is too tight it can be the pivot point for the shoes which will give a spongy brake pedal for a short while until the cable fails.

A really bad or loose front wheel bearing will also cause the pedal to need to be pumped after driving it even a short distance. The wheel will wobble which allows the rotor to push the caliper piston back in which then needs a pump or two of the pedal to fill back up again.
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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
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
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Unread 02-18-2010, 06:49 AM   #27
Schne
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Well I guess I will try to bench bleed the master again. I did adjust the rears, before I tried anything else. And I don't drive it for the presure to disapear., It does it when I am just sitting still. Thanks again for the advise.
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Unread 02-18-2010, 06:58 AM   #28
Mike Romain
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Quote:
Originally Posted by Schne View Post
Well I guess I will try to bench bleed the master again. I did adjust the rears, before I tried anything else. And I don't drive it for the presure to disapear., It does it when I am just sitting still. Thanks again for the advise.
Good luck man! I hate brakes that won't adjust/bleed. When I did my last frame off under a tent at -35C and had the moving truck already loaded and gone the last thing was the brand new brakes and they wouldn't bleed. Nothing would come out of the rears. I finally popped the combo valve by my helper pumping up the pedal hard while I popped the front bleeder. This allowed it to switch thinking it had a front leak. It then still took 3 or 4 runs around all the bleeders to get the pedal up and the last of the air bubbles out.

Also be sure you get the bleeders closed before the helper lets up on the pedal or it can suck air back into the cylinder. This is why they recommend a clear hose off the bleed nipple going into a jar with some fluid in the jar to start so it can't backfeed air.
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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
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
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Unread 02-23-2010, 06:21 AM   #29
Mike Romain
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Did you get a chance to do a second bench bleed of the MC? As mentioned, mine was a bear to bleed so the piston wouldn't go in more then the 1/8" called for. It took long enough I didn't think I was going to get it... If the piston goes in deeper than that 1/8" before it comes up real hard on the bench bleed, you still have air in the MC.

When you are wheel bleeding it, is air still coming out?
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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
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
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Unread 02-23-2010, 11:25 AM   #30
Schne
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I got a litle air out of the fronts, after jacking up the back of the jeep as far as my jack would allow. And I didn't have time to bench bleed yet, I will try that thursday night, after work. Thanks for the info.
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