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Drum Brakes Questons


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I did a complete brake overhaul a few weeks ago, although the braking did improve they were still a spongy but haven’t paid that much attention to them until I started pulling a trailer. 

 

I was looking through the FSM and learned I had skipped a step by not doing the bleeding procedure for the by-pass differential valve. 

 

The procedure stated that with one of the front bleeder fittings open and after depressing the brake pedal that the brake warning light on the dash should come on. My light never came on, it works beause if I read correctly it’s the same light as the emergency brake light which works when they are engaged. 

 

The brakes still feel spongy after the procedure, did it step by step and then rebled everything, I used a vacuum pump for bleeding purpose. 

 

Am I missing something? 

 

Also, I’m attaching 2 pics, 1 of FSM drum schematic and 1 from the driver’s side drum assembly.  If you look at what I have and the FSM drawing the adjuster and adjuster cable are opposite. I put them back together exactly the way they came apart, even the brake adjuster clip has an “L” on it.  Is mine backwards??

 

095AE85B-5CFE-43C3-B59C-ACB86AA90C26.jpeg

88090183-B36B-48E0-A9A0-3A8A594462D3.jpeg

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Update::

Got to looking closer and noticed there was hardly any noticeable wear on the drum pads, Uh?? Thought I had adjusted them out tight the first go around, evidently I didn’t. Adjusted them until I couldn’t get the tire to turn and backed off a little, did both sides, 100% better now. 

 

Also noticed quite a bit of wear on the drum. Initially I didn’t think it was too bad but this may also be part of my problem. Don’t have any way to mic it so may take one and have it mic’ed or just replace both of them. 

 

Still curious about the light not coming on and the installation setup I mentioned on my initial post. 

 

Any thoughts or ideas appreciated. 

 

Thanks. 

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43 minutes ago, Griff04 said:

Still curious about the light not coming on and the installation setup I mentioned on my initial post.

Good chance the FSM is only showing the exploded view for the right rear drum.

 

Three (3) conditions will light the brake light bulb.

  1. Parking brake switch
  2. Ignition switch during CRANK (bulb check)
  3. Brake warning switch (unequal brake pressure)

If brake light lit during 1 & 2, then either there wasn't enough unequal pressure to shift the valve, a stuck valve or a faulty Brake Warning Switch.
 

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25 minutes ago, Ωhm said:

Good chance the FSM is only showing the exploded view for the right rear drum.

 

Three (3) conditions will light the brake light bulb.

  1. Parking brake switch
  2. Ignition switch during CRANK (bulb check)
  3. Brake warning switch (unequal brake pressure)

If brake light lit during 1 & 2, then either there wasn't enough unequal pressure to shift the valve, a stuck valve or a faulty Brake Warning Switch.
 

Definitely lighting during 1&2 

 

You will probably save me some time looking for it, where exactly is the brake warning switch ilocated??

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7 hours ago, Griff04 said:

The procedure stated that with one of the front bleeder fittings open and after depressing the brake pedal that the brake warning light on the dash should come on. My light never came on, it works beause if I read correctly it’s the same light as the emergency brake light which works when they are engaged. 

 

The brakes still feel spongy after the procedure, did it step by step and then rebled everything, I used a vacuum pump for bleeding purpose. 

 

Am I missing something? 

 

 

Not certain, but I don't think the vacuum pump can bleed the bypass circuit.

 

The reason for the special procedure is that the MJ has two brake lines to the rear. The primary goes through the rear load/height sensing proportioning valve. The secondary line is a bypass, so that if you lose the front brakes you'll get full braking to the rear, not "proportioned" braking. The bypass circuit isn't open unless the shuttle valve in the front distribution block has been tripped. That's why the FSM calls for opening a front bleeder and stepping on the brakes. The idea is to simulate a front brake circuit failure. If you didn't step on the brakes hard enough with the bleeder open, perhaps you didn't trip the shuttle valve.

 

Also, the fact that your brake warning light on the dash illuminates doesn't mean the switch at the distribution block is working.

 

Here's what the distribution block looks like, with the bypass circuit opened by tripping the shuttle valve:

 

image.jpeg.3fc2edae86d7bf90f96c4a2dbafc6112.jpeg

 

That pastic thingie on top is the switch that activates the warning light. Normally the plunger rests in that notch, which is the OFF position. When it moves to either side (depending on whether the front or rear circuit fails), the plunger is pushed up and the light goes on.

 

If you didn't trip the plunger, the bypass circuit would be closed and the suction bleeder wouldn't pull anything through the bypass circuit. Even with the plunger tripped, I don't know how effectively the vacuum bleeder would get the bypass circuit, since the primary circuit is also open. I use a vacuum bleeder on XJs, buty I still use the foot pedal when bleeding MJs.

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10 hours ago, Eagle said:

 

Not certain, but I don't think the vacuum pump can bleed the bypass circuit.

 

The reason for the special procedure is that the MJ has two brake lines to the rear. The primary goes through the rear load/height sensing proportioning valve. The secondary line is a bypass, so that if you lose the front brakes you'll get full braking to the rear, not "proportioned" braking. The bypass circuit isn't open unless the shuttle valve in the front distribution block has been tripped. That's why the FSM calls for opening a front bleeder and stepping on the brakes. The idea is to simulate a front brake circuit failure. If you didn't step on the brakes hard enough with the bleeder open, perhaps you didn't trip the shuttle valve.

 

Also, the fact that your brake warning light on the dash illuminates doesn't mean the switch at the distribution block is working.

 

Here's what the distribution block looks like, with the bypass circuit opened by tripping the shuttle valve:

 

image.jpeg.3fc2edae86d7bf90f96c4a2dbafc6112.jpeg

 

That pastic thingie on top is the switch that activates the warning light. Normally the plunger rests in that notch, which is the OFF position. When it moves to either side (depending on whether the front or rear circuit fails), the plunger is pushed up and the light goes on.

 

If you didn't trip the plunger, the bypass circuit would be closed and the suction bleeder wouldn't pull anything through the bypass circuit. Even with the plunger tripped, I don't know how effectively the vacuum bleeder would get the bypass circuit, since the primary circuit is also open. I use a vacuum bleeder on XJs, buty I still use the foot pedal when bleeding MJs.

So one thing the FSM doesn’t mention is when opening the front bleeder and then depressing the brake pedal should the pedal stay depressed during the rear bleeding?? I actually engaged it twice when the light didn’t come on and did not keep it depressed during the rear bleeding. 

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On 7/2/2018 at 8:17 AM, Griff04 said:

So one thing the FSM doesn’t mention is when opening the front bleeder and then depressing the brake pedal should the pedal stay depressed during the rear bleeding?? I actually engaged it twice when the light didn’t come on and did not keep it depressed during the rear bleeding. 

 

No, the peddle doesn't stay depressed ... but the front bleed screw remains open.

 

Obviously, if you're bleeding the traditional way (by having one person open and close the bleeder while a second person alternately depresses and releases the brake peddle) the peddle can't stay depressed.

 

The purpose of opening a front bleed screw is only to simulate a failure of the front brake circuit (loss of pressure) and to trip the shuttle valve. If the shuttle valve doesn't move, the bypass circuit doesn't open and you can't bleed the bypass circuit.

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9 hours ago, Eagle said:

 

No, the peddle doesn't stay depressed ... but the front bleed screw remains open.

 

Obviously, if you're bleeding the traditional way (by having one person open and close the bleeder while a second person alternately depresses and releases the brake peddle) the peddle can't stay depressed.

 

The purpose of opening a front bleed screw is only to simulate a failure of the front brake circuit (loss of pressure) and to trip the shuttle valve. If the shuttle calve doesn't move, the bypass circuit doesn't open and you can't bleed the bypass circuit.

 

I’ll give it another try using the 2 person method. Need to get the new drums on it before going back through it. Trying to remember what size brake shoes I put on it 1 3/4 or 2 1/4 so I can order them. Looks like I’m going to have to pull the drum back off to measure them. 

 

Thanks for your replies. 

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Looking at your photo, it appears that both brake shoes have the same length lining. Usually there are two with long linings and two with shorter linings. They get paired, one long and one short per side. The short lining goes toward the front of the vehicle.

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57 minutes ago, Eagle said:

Looking at your photo, it appears that both brake shoes have the same length lining. Usually there are two with long linings and two with shorter linings. They get paired, one long and one short per side. The short lining goes toward the front of the vehicle.

 

That’s interesting and definitely something I need to look at. So that would mean I would have put both shorts on one side...uh??

 

Thx...

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On 7/2/2018 at 9:13 PM, Eagle said:

Looking at your photo, it appears that both brake shoes have the same length lining. Usually there are two with long linings and two with shorter linings. They get paired, one long and one short per side. The short lining goes toward the front of the vehicle.

 

I got done with the RMS replacement and throttle body cleanup and cicled back to the brakes. 

 

Good eye man, you were here absolutely correct, I have both shorts on the passenger side. What a dumba&$ I am, didn’t even pick up on the different sizes when I replaced them. If I worked for me I’d fire myself. 

 

Anyway, had planned on replacing the drums due to the wear on them so I’ll do both at the same time. Once I get the pads on correctly and new drums on I’ll go back through the bypass circuit bleed again and see how the brakes do then. 

 

Griff04

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Well I got the shoes installed correctly along with the new drums on this morning. Man, the new drums were tight going on. Had the adjusters out about 3 turns each and had to bump the drums on, they were dragging so badly after getting everything together I wound up with only 1 turn on the adjusters. 

 

They still feel a little spongy so I’ll go through a little more adjusting and the bleeding process in the morning. 

 

Right now  I’m going to bed, it was a long nite at work. 

 

Griff04

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Things shouldn't be tight going together. Your emergency brake cables may need to be adjusted. Hopefully they are not frozen. I had that happen once, made trying to put the drum on that side a bear/impossible with new shoes.

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So I went back at it again this morning bleeding the brakes manually. Went through the entire procedure including bleeding the bypass system. Never did get the dash light when doing this so not 100% sure that it actually went to bypass mode with the front bleed fitting open and bleeding the rears per the FSM. Maybe the switch is not working on the bypass unit sending a signal to the light???

 

The brakes do feel better but I don’t think they are 100%, still have to give them 1 small pump before the pedal really feels tight. Maybe I’m just used to the full disc brakes on my GMC 2500. 

 

I’ll do the weekend trip to the hunting camp and see how it goes. There may be some “more meat left on this bone” before it’s completely resolved. 

 

Appreciate eveyone’s input. 

 

Griff04

 

 

 

 

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