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Master Cylinder Outputs

Discussion in 'Intermediate CJ-5/6/7/8' started by officer_friendly, May 11, 2020.

  1. May 11, 2020
    officer_friendly

    officer_friendly Member

    Northern California
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    I have done lots of searching on this topic but have not found a definitive answer, so I'm hoping I can confirm this here. I have a 74 CJ5 that originally had front drum brakes. I swapped to disc brakes from a 75 Chevy K10 in the front, and kept the stock drums in the rear. I used a disc/drum proportioning valve for a K10.

    When I hooked everything up and bled the brakes, I found that my stock master cylinder did not push enough volume for the front discs. So I bought a MC for a 76 CJ5 that would have been disc/drum. Here's where I suspect my problem has occurred:

    I connected the hard line for the front disc brakes to the front of the master cylinder (towards the front of the vehicle), and the hard line for the rear drums to the rear of the MC (closest to the driver). On a short test drive I had a very firm pedal, and the jeep stopped well. However I noticed a buildup of pressure in the front discs that was causing a noticeable resistance on the jeep. When I pushed the clutch in trying to coast, the jeep slowed down pretty fast without applying any brakes. After a short 3 mile drive I returned home and measured the temperature of the discs, they were 160 degrees F, which seems hot for a quick drive and only using the brakes a few times.

    From what I have read it seems that I have the lines swapped, and the front discs line should be connected to the rear outlet on the MC, closest to the driver. But I haven't found any photos or anything to prove this is the correct routing. It seems there may be a drum pressure valve in the front outlet that keeps the drums pressurized, but in my case is causing excess pressure in the discs? Does anyone know the answer to this for sure? I can take some photos tomorrow, and if anyone could provide a photo of their 76+ master cylinder that would be helpful. Thanks.
     
  2. May 11, 2020
    BadGoat

    BadGoat How High Can You Climb?

    Northern Virginia
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    In general, for a disc/drum setup the disc brakes are connected to the larger of the two reservoirs. This is often the reservoir closer to the firewall. Also, check the master cylinder to see if it has a residual pressure valve in it. They are sometimes installed where the brake line bolts up. If so, you shouldn't need the external proportioning valve.

    Mike
     
    Last edited: May 11, 2020
    Lockman likes this.
  3. May 11, 2020
    IrishCJ6

    IrishCJ6 Member

    Wicklow Ireland
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    Front brakes, port closest to firewall
     
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  4. May 11, 2020
    sterlclan

    sterlclan Member 2022 Sponsor

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    And ps a 76 has drums.
     
  5. May 11, 2020
    officer_friendly

    officer_friendly Member

    Northern California
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    On this master cylinder both reservoirs seem identical in volume, but I swapped the lines anyways just in case. Bled the system and no difference, drums seem to work fine but discs in the front hold pressure and drag badly. Both discs hold pressure evenly and if I crack the bleeder on either side it releases the pressure. However once I press the brake pedal 2 or 3 times they are locked up again.

    Then I took the MC apart to see if it has any residual pressure valves. I can push a toothpick fully through both outlets, indicating there are no pressure valves. Then I removed the MC piston and inspected, there does not seem to be anything that would store residual pressure in that either.

    My understanding is that the J10, Wagoneer, and CJ5 all used the same MC in 76. And the J10 and Wagoneer had front D44’s with disc brakes which were nearly identical to mine. I’m thinking the proportioning valve I used must somehow be causing this. Can the prop valve hold pressure?
     
  6. May 11, 2020
    Dphillip

    Dphillip Member

    Omaha NE
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    I installed Chevy truck disc brakes on the front of my CJ but I have a narrowed Dana 44 axle from a Wagoneer.
    When I put on the disc’s, I also had some issues sorting out the proportion valve pressure between the front and rear brakes. After plenty of frustration I just bypassed the prop valve and ran the line straight from the master cylinder to the front disc brakes. The rear brakes, I did the same no prop valve but I did install an aftermarket inline 2 pound residual valve. After omitting the prop valve the brakes were perfect. I’ve since put disc brakes on the rear and removed the residual valve and installed an aftermarket inline adjustable prop valve. I’ve got the pressure to the rear turned down some to keep them from locking up.
    I’m not recommending you ditch the prop valve but that’s what I did or as you mentioned your valve could be holding the pressure.

    Good luck.
     
    Last edited: May 11, 2020
  7. May 11, 2020
    officer_friendly

    officer_friendly Member

    Northern California
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    Dphillip what year is your CJ and what MC did you put in yours?

    I think I have narrowed this issue down to the MC, although I cannot see the residual valves it is not allowing fluid to return to the reservoir. If I turn open the brake line fitting at the rear MC outlet, pressurized fluid spurts out and then my front brakes unlock. This tells me the fluid being held at pressure up there. I’m thinking about picking up a 1980 CJ5 MC, thoughts?
     
  8. May 11, 2020
    '74Renegade

    '74Renegade Active Member

    Fair Oaks, CA
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    If your 76+ master cylinder was intended for a drum/drum system, I suspect that there is a 10lb residual valve in there.
     
  9. May 11, 2020
    officer_friendly

    officer_friendly Member

    Northern California
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    There must be a residual valve in the MC I have, I’m going to order an 80 CJ5 MC since my research says they all had disc fronts.
     
  10. May 11, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Right, 1977 was the first year for disks in CJs. And they were optional for one or two years, so a '77 or '78 could also have 4-wheel drums.

    I presume this is manual brakes? There is a '77/78 master cylinder for disk/drum that's manual brakes and was used with the 11"x2" rear drums. It's an 812 7820.

    RockAutoMC.png

    If you want, you could try your local Olde Tyme Parts Store and see if they can get the Raybestos number or cross to the Jeep PN.

    Jeep changed to 10"x1.75" drums for the rear in mid-78. A MC for manual brakes with disk-drum would then be a 5358745 or 5359552.
     
    Last edited: May 11, 2020
  11. May 11, 2020
    Dphillip

    Dphillip Member

    Omaha NE
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    My CJ is a 1976

    When I installed the Chevy truck disc brakes I used a master cylinder for a CJ with disc brakes.

    When I went with rear disc brakes I used a master cylinder for Chevy truck with 4 wheel disc brakes.
     
  12. May 11, 2020
    BadGoat

    BadGoat How High Can You Climb?

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  13. May 11, 2020
    officer_friendly

    officer_friendly Member

    Northern California
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    Good catch on the 11x2 drums changing after 79. I have manual brakes, trying to keep things simple. I went ahead and ordered a 79 MC from the local parts store. I’ll install tomorrow and update if it fixed the issue or not.
     
  14. May 11, 2020
    Fireball

    Fireball Well-Known Member 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    It's possible the master cylinder isn't traveling all the way back to open up the return ports. Make sure there is a little slop between the rod from the pedal and the master cylinder piston when the pedal is released. Usually you want 1/2" or so of free travel at the pedal before the piston starts to move.
     
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  15. May 11, 2020
    bigjohn

    bigjohn Active Member

    Kelso Wa
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    Are you running the proportioning valve with a reset button?
     
  16. May 12, 2020
    officer_friendly

    officer_friendly Member

    Northern California
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    The proportioning valve has a little button on the front. I'm using a GM style PV2 proportioning valve. I disassembled the whole proportioning valve, cleaned it and reset it. I also installed the tool that holds the valve in the center for bleeding the brakes.
     
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  17. May 13, 2020
    officer_friendly

    officer_friendly Member

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    I installed the 79 MC today and bled everything. I pumped up the brakes and the discs could still be turned by hand, although the drag was a bit more than I would expect. I went for a good 30+ minute test drive and could not notice any feeling of drag. I took the temperature of the brake rotors with an IR thermometer and they were only 75 F, so I think everything is working as it should. Jeep stops very well. Previously the rotors were almost 200 F after a short drive.
     
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  18. May 13, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Normally the runout of the rotor and bearings will push the pistons back a bit, so I would not worry about some rubbing after application, as long as they turn by hand. Good deal!
     
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