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  #1  
Old November 4th, 2019, 09:49 AM
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Default Brakes Locked up?

On a test drive yesterday, my brakes locked up. I posted here because the Master Cylinder (MC) isn't stock for the M715 it's from a J10 or J20 Pickup.

The MC is new (not rebuilt). The wheel cylinders are new (not rebuilt). The lines are mostly original including the rubber flex lines. The brake fluid was new in May 2019.

The truck has been sitting since July 2019 (I had engine issues).

The brakes locked after driving 10 miles, and when I stopped and applied parking brake the brakes locked up. I had to bleed them a bit to release the pressure.

Some suggested that the free play between the MC plunger and the brake pedal lever needs adjustment (blocks the return fluid inlet when the fluid gets hot and expands). Others have suggested that the return line inlet in the MC might have crud from the old brake lines. One person said that the wheel cylinders could have condensation in them and the return spring couldn't push the cylinder back to its original position.

I didn't check if the brakes were locked on just one wheel or on all wheels (It felt like all wheels were locked though). I didn't have a jack or jack stands with me so I could check, and I was stopped on the road! Not a good place for maintenance*.

Anybody have other ideas or can shed a little light on this for me?

I have been asked to drive in the Veteran's Day Parade, but I can't (won't) do it until I fix these brakes. Stopping is Boucoup important!
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Old November 4th, 2019, 01:06 PM
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Time for a complete brake system overhaul. I would replace the rubber lines with new lines (not NOS, which are 50 years old). Then use DOT5, it will last longer and perform better than DOT3.

The rubber lines deteriorate with age and can swell or have chunks come loose inside the lines plugging them.
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Old November 5th, 2019, 06:30 AM
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Anyone have a good idea for the exact items to replace the stock rubber hoses to the front and rear brakes? Photos could be helpful. Better yet, how about a URL?

I conducted a search for jeep brake lines and I couldn't tell if any of those would work. They don't give measurements or thread counts, etc.

If I don't hear back, I guess the only thing I can do is go to auto parts stores with the rubber brake lines and ask if they have something comparable, or I can buy NOS.
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Old November 5th, 2019, 08:46 AM
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Try ORD in colorado (offroad design). The make custom brake lines and probably know what fittings work with stock m715. If you have the lines already removed even better, since you can measure them.

https://www.offroaddesign.com/index.htm

or BJ's offroad.

http://www.bjsoffroad.com/
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Old November 5th, 2019, 09:47 AM
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Great, Thanks!

I found this site too, and there is a kit for the M715, but to find it, you will need to use the search feature:

https://classictube.com/products/
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Old November 5th, 2019, 02:13 PM
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I bought the rubber hoses and other brake hardware from Memphis Equipment.
http://www.memphisequipment.com/M715...rake-Hose.html
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Old November 5th, 2019, 08:33 PM
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I bought from Memphis Equipment too. Worked great.
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Old November 6th, 2019, 05:58 PM
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Weird that when you applied the parking brake that the wheel brakes locked up...the stock parking brake is on the rear of the tcase and doesnt share anything with the fluid/wheel mounted brake system...

Maybe i am missing something...
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Old November 7th, 2019, 02:05 PM
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I think the hydraulic brakes just locked up because there is either a clog in the Master Cylinder return fluid inlet hole, or the master cylinder rod and the pedal interface needs adjustment. The fluid just got hot and expended I think.

I am going to drain the fluid Friday and replace one section of steel line (kinked a bit) and all three sections of rubber line. Then I will put in new DOT 4 and try to duplicate the lock up. Dot 5 has a different chemical composition and will leak in places that DOT3 and DOT4 will not. DOT4 (I am told) is for rubber brake lines.

If I can't duplicate the lockup, then I will go to the parade. If the brakes lock up again, I will need to replace the entire brake system and I will miss the parade this year.

I am prepared for either outcome.
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Last edited by plumas.placer.miner; November 7th, 2019 at 02:16 PM.
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Old November 8th, 2019, 05:58 AM
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Good luck, I hope you get to drive in the parade this year.
As for the Dot 5 fluid. If you should decide to replace the entire brake system (highly recommended) then you can use it.

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