Brakes kicking my butt

1950 - 1952, M38, questions, discussions, regarding anything related to the M38.
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Scott in NM
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Brakes kicking my butt

Post by Scott in NM » Wed Jun 14, 2017 5:54 pm

Got air into my brake system. Bled brakes (no leaks noted at wheels or master cylinder prior to this). Have bled brakes through at least four rotations between all tires. Brake system appears to be restored when pumping, but let sit 10 seconds and pedal sometimes goes to the floor (note the sometimes, doesn't happen all the time). System performance sometimes normal, then without touching it pedal goes to the floor. No leaks noted. Seems like an air bubble that just doesn't want to come out but has no problem moving around to good and bad spots. All cylinder seem OK. Suggestions? Have important car show this weekend and really don't want to tow it there. Help please!

Scott

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D Pizzoferrato
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Re: Brakes kicking my butt

Post by D Pizzoferrato » Thu Jun 15, 2017 4:39 am

Assuming (there's that word) that you have bled these in the correct order, it could be as simple as brake shoes needing adjusted or one or more shoes out of adjustment. if one or more shoes are too far away from the drum, it will require multiple applications (pumping) of the pedal to extend the pistons to fill the void between the shoe and drum.
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artificer
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Re: Brakes kicking my butt

Post by artificer » Thu Jun 15, 2017 11:40 am

Go through methodically & as David suggested this may & very likely is an adjustment problem.

Jeep bleeding is easy, simple & straightforward plus will be successful no matter what order one uses [recommended longest to shortest, is just much quicker & less wasteful] provided the master cylinder is always kept full while bleeding.

The master cylinder is full, now?
Have you used DOT 5 or standard DOT fluid?
I never recommend the pumping method & DOT 5 takes longer to settle out if the fluid becomes aerated.

How to tell whether the problem is adjustment or air is very simple as well.

ADJUSTMENT
Pump the pedal quickly & the pedal will rise to be a good HIGH HARD pedal after progressive pumps.
As soon as one let's the pedal fully off the next application will go back to the original position maybe near the floor & require exactly the same procedure to get a full hard pedal again.

The reason this happens is because the rapid pedal pumping causes the brake shoes to be adjusted out to the drum hydraulically.
@ each pump all the fluid can't get back to the master cylinder reservoir quickly enough so each application gets more fluid into the wheel cylinders ratcheting out the brake shoes.

When the pedal is released long enough all this fluid returns to the reservoir so the hydraulic adjustment causing this high hard pedal is lost.

AIR
Will never pump right up as adjustment does & always feels much softer & has a very spongy feeling.

With the twin bottom adjustable anchor system adjustment is not easy whereas the later model shoe sliding bottom anchor system is not such a pain.

Following instructions in this link will help ensure adjustment is correct, as simply as possible:
http://jeepdraw.com/images/BRAKES-101.pdf
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
TO DIAGNOSE, TROUBLESHOOT OR FAULT FIND ANY AUTO SYSTEM....
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.

Scott in NM
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Re: Brakes kicking my butt

Post by Scott in NM » Fri Jun 16, 2017 7:53 am

Gentlemen,
Thanks for excellent posts! Brake performance is exactly as Artificer describes, and it is always redeeming when both inputs agree. Gives me an excellent place to start.
Also, articles like the one Artificer contributed to the Jeepdraw board are always appreciated by those of us who have knowledge, but never enough. I'd like to thank ALL those that contribute to this board and Jeepdraw as your help is what allows us schmucks to continue the hobby and perhaps learn something along the way.

And when I take this vehicle to the biggest car show in our area on Sunday, my visitors will have you to thank. Image



Scott

Scott in NM
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Re: Brakes kicking my butt

Post by Scott in NM » Fri Jun 16, 2017 2:43 pm

OK, I've spent all day with th!@#$% brakes. Adjusted them per the TO and per Artificer's method. Neither works. Depress half way down quickly 3-4 times, pressure builds up but maintain that pressure and pedal goes to the floor over 3-4 seconds. If I apply gentle pressure to the pedal, no brake pressure is apparent and pedal goes to the floor. No leaks, even peeled back the rubber cups on the wheel cylinders to check for fluid.

Unless someone has other ideas, I'm down to repair by substitution. Still willing to take inputs though.

Scott

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Re: Brakes kicking my butt

Post by artificer » Fri Jun 16, 2017 3:17 pm

If when one has pressure & a foot on the brake pedal, then the pedal sinks to the floor, the master cylinder primary cup is damaged/leaking.
The master cylinder needs overhaul/or replacement, if the bore is corroded.

Remember I said I don't go for pump bleeding. [particularly on non new, older systems]
If the pedal is sinking down the above indicates precisely why pumping is NOT good.
When pump bleeding, if the primary seal traverses a corroded/pitted master cylinder bore, that it doesn't travel over normally, the seal becomes damaged.
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
TO DIAGNOSE, TROUBLESHOOT OR FAULT FIND ANY AUTO SYSTEM....
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.

Scott in NM
Sergeant Major of the Gee
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Re: Brakes kicking my butt

Post by Scott in NM » Sun Jul 16, 2017 7:40 am

As a final conclusion to all who have read the previous posts - I finally got around to rebuilding the master cylinder, and all works now better than new. Don't know which part was bypassing pressure, but situation now resolved. Hope others have learned from this thread, I know I do when I read those of others. Thanks everybody!

Scott

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