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Unread 03-15-2009, 11:11 AM   #1
SuB8HaVeN
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Brake conversion issues with discs on Willys Dana 25

I just got done yesterday pulling the knuckles off the axle, and cleaning and painting everything and putting it all back together. I got new calipers from Napa, and the brake lines and banjo bolts and all hardware are new. I bled all the brakes and the pedal feels great. The calipers still are not releasing all the way. I took it for a spin around the block really quick and got up to about 40mph and when I got back I could smell the brakes burning. It will roll, so they're not entirely seized up, they just aren't releasing enough to let the wheels spin free. If I jack the front end off it takes some effort to turn the wheels with the brakes not applied. The brakes work amazing, they will put you in the windshield in a heartbeat, but I still think I have an issue.

The only thing left I can think of that is causing the issue is the master cylinder. Everything else is new, they bleed just fine and everything. Is there any sort of adjustment or anything I need to fool with, or is it time for a new MC? Napa has them for $60 in stock so I'm in luck. Thanks in advance for any info and help.

Edit: To add to the mystery, I tried adjusting the plunger rod or whatever it's called, according to my factory service manual. I have it adjusted almost all the way out and they are still not releasing completely. I took it around the block again and the rotors were HOT again. The way I have the rod adjusted now you have to press the brake pedal about 2-3 inches before engaging the MC.

I'm beginning to point at the MC having crap clogged in it? What do you guys think?

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Last edited by SuB8HaVeN; 03-15-2009 at 03:45 PM..
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Unread 03-15-2009, 10:27 PM   #2
jeepdaddy2000
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Most drum MC's are designed to hold a couple of pounds of pressure in the lines to help keep the wheel cylinder cups from collapsing. While this is good for drum set ups, it will not work for disks. You will have to swap the MC for a disk brake application.
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Unread 03-16-2009, 03:26 AM   #3
oilcan
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You can pull the check valve out of your m/c. There was a disc conversion writeup in JP a while back, and there might be one here, too.




This is how it goes as far as I can remember it. For goodness sakes look it up first, because my memory ain't what it used to be!

Pull the brake line to the front from the m/c
Run a wood screw into the seat (try not to damage it)
Remove the check ball (and spring?)
Drive the seat back in
Put the brake line back on
Done
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Unread 03-16-2009, 07:17 AM   #4
BioTex
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I copied this from earlyCJ5.com website.

Remember that if you are using a dual master cylinder from a drum-brake vehicle, you will need to remove the 10-lb. residual-pressure valve from the front port of the MC and plumb a 2-lb. unit inline to the front brakes I used a Wilwood PN: 260-1874. (The photo at left is from my incomplete installation). If you don't remove that 10-lb. pressure valve, the front brakes will drag, as there is no spring to return the calipers like there is in a drum installation. If you just remove the stock valve and don't install a 2-lb. valve like I did, you may end up having to double-pump the brakes all the time to get any pedal, as there is nothing to stop too much brake fluid from flowing back to the master cylinder when you release the pedal.
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Unread 03-16-2009, 07:59 AM   #5
SuB8HaVeN
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Wow, it's always the little things that kill me. It looks like I'll be removing the master cylinder to remove the check valve. I also neglected to pull the calipers back and forth to make sure there was still enough clearance when they move so they don't hit the knuckle. I didn't realize these GM calipers are the "floating" style.

I'll probably get to this later this week if I can get some time off work. I'm planning on pulling the entire front clip off so I can get to the steering box and MC easier, and take care of some rust along the way. It is a PAIN having it mounted to the frame rail!

Thanks for the help though guys, it looks like I've found my problem.
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Unread 03-16-2009, 07:09 PM   #6
jeepdaddy2000
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If you are running the original single master cylinder I would suggest swapping to a dual setup designed for disks. You can use a later CJ model if you want.
The dual MC is a much safer system and will allow residual pressure to stay in the rear system.
It will require some minor re-plumbing to split the systems but is easily doable.
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Unread 03-17-2009, 03:09 AM   #7
oilcan
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X2............
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Unread 05-29-2009, 03:10 PM   #8
SuB8HaVeN
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OK, I had to dig this up because this is next on the list as soon as I can pull the steering box to get it out of the way. Do I HAVE to have a metering valve and/or proportioning valve when I run this disc front, drum rear setup? I crave simplicity, but it's also important to have the brake system operating.

I've considered going with a dual chamber master cylinder but can't find an application that will fit like my frame mount is since I don't want to deal with the complications of mounting it somewhere else or on the firewall. The later CJ dual chamber master cylinders all mount on the firewall.

I also was talking with an old mechanic at the local Jeep shop while ordering some parts for the CJ and new style hubs for the wagon, and he didn't seem to think there was a check valve inline in the master cylinder as the Earlycj5.com website suggests. Has anyone seen this check valve in person?
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Unread 05-29-2009, 03:24 PM   #9
CSP
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I don't know about the early, single reservoir masters, but on the dual reservoir the residual pressure valve is a simple spring and rubber plug behind the inverted flare cone on the line output.

There has to be something there or the springs in the drums would pull the linings too far from the drums. You'd step on the brakes and there wouldn't be enough stroke to push them back against the drums.
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Unread 05-29-2009, 03:40 PM   #10
SuB8HaVeN
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That makes sense, I guess I just need to take it apart and see what's inside of that fitting where the 2 brake lines come out of the master cylinder.

I'm thinking if you run a single reservoir master cylinder, you don't need a proportioning valve because that single cylinder is feeding both at the same time. Does that make any sense?
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Unread 05-29-2009, 04:08 PM   #11
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There should be a residual pressure valve in the outlet of the master cylinder that supplies the front brakes (disc brakes do not need it). Run a screw into it, pull it out, then re-attach your brake line. There may be a small brass fitting (the seat) that comes out as well. There should be some form of washer/spring/ball in there that holds the pressure and needs to come out.
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Unread 05-31-2009, 06:58 AM   #12
jeepdaddy2000
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The swap to a dual MC should be fairly simple, as 70 and 71 Cj's used these under the floorboards. I replaced my original dual with a 73 drum application and it bolted right in. The only issue I had was the lines were a different size, forcing me to install reducers to adapt to the existing system.
If memory serves, there may be a bit of a difference to the actual single cylinder mount which may need to be addressed, but to be honest, I can't remember.
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Unread 05-31-2009, 05:27 PM   #13
SuB8HaVeN
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There is a difference with the mounting, that's my issue with it. Kaiserwillys.com sells adapter kits but wants an unholy amount of money for a few pieces of metal.
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