Just won an auction - M1088A1

[GeneralDisorder]

Wow. Have not encountered that. Can you disassemble and inspect/post pics of the adjuster? Would like to understand this failure.

Some of mine were pretty UGLY with rust and failed seals but it all functioned fine after cleaning, lubing and adjusting. Let me find that pic.......

 

[Ronmar]

Well the adjusters only cycle when the actuators cycle the brakes… That sounds weird… is the drum out of round? Take the wheel off, put on some lugs to hold the drum in proper position and spin it...

 

[GeneralDisorder]

Well the adjusters only cycle when the actuators cycle the brakes… That sounds weird… is the drum out of round? Take the wheel off, put on some lugs to hold the drum in proper position and spin it...

Perhaps the threads are messed up allowing it to slip out but not back in due to a non-axial force.

 

[Ronmar]

Perhaps the threads are messed up allowing it to slip out but not back in due to a non-axial force.

If I recall, the spring loaded teeth(with sawtooth serrated faces) are pushed out of the way and ride over the spiral sawtooth grooves on the plunger as the plungers travel outward(pushed by the wedges) because the two sets of sawtooth ridges meet angle to angle(like teeth on a ratchet). Then as the brakes retract and the plungers slide inward, the saw tooth grooves on the plunger and the adjuster teeth ridges engage vertical to vertical(again like teeth on a ratchet), which forces the plunger to rotate and add a small turn to the adjuster screw which is held by the manual adjusters on the end. So the plunger assembly gets a little longer with each brake application…

i seem to recall it being possible to install the teeth incorrectly so the angles and flats did not line up correctly and would not engage properly to cause the above action, disabeling the auto adjuster, so brakes would get looser and looser.

that and damaged manual adjusters would probably only show up after driving a bit though, and it does not sound as if it has been really driven since the brake job? So locking as you roll It out of the bay after work points me back to a out of round drum? You set the shoe spacing on the wide part of the drum and as it rotated to the narrow axis, it engages the shoes?

 

[dwlindsey]

Wow. Have not encountered that. Can you disassemble and inspect/post pics of the adjuster? Would like to understand this failure.

Some of mine were pretty UGLY with rust and failed seals but it all functioned fine after cleaning, lubing and adjusting. Let me find that pic.......

I put the adjuster in a vice and spun it with a pair of channel locks. There were a couple of tough to turn spots, which got better as I worked it. It's behaving well, I think. I'm not sure I trust it enough to re-install it.

I can't explain what happened, I'm sure I had it adjusted correctly yesterday and it was fully extended today.

 

[dwlindsey]

If I recall, the spring loaded teeth(with sawtooth serrated faces) are pushed out of the way and ride over the spiral s it It does not sound as if it has been really driven since the brake job? So locking as you roll It out of the bay after work points me back to a out of round drum? You set the shoe spacing on the wide part of the drum and as it rotated to the narrow axis, it engages the shoes?

Correct. I adjusted the brakes yesterday and tried to back it out of the garage today.

I set the shoe gap at the bottom of the drum, in the space where the inspection gap is. I had it set to 30 thousandths. I did not apply the service brakes and check it again. I'll do that next time.

This worked on the intermediate axle brake, but not on the rear one.

I really can't imagine that I turned the adjuster all the way out and then confirmed the 30 thousandths gap. I had the gap at 60 thousandths, 15 thousandths and then back to 30 . . . it felt right to me yesterday, but was really wrong today.

 

[dwlindsey]

Well the adjusters only cycle when the actuators cycle the brakes… That sounds weird… is the drum out of round? Take the wheel off, put on some lugs to hold the drum in proper position and spin it...

I did apply the brakes several times at the start of backing out. I agree it sounds weird.

The wheel is off, the front canister is off, the brake shoes are off.

When I get it back together, I'll try to check the drum and make sure it's round. I don't imagine that's the problem, but I'll try to check it.

 

[Ronmar]

I guess another possibility is that the spring brakes did not fully release with applied park air? Wonky actuator or wedge? The operation is the sum of all the parts…

 

[GeneralDisorder]

I guess another possibility is that the spring brakes did not fully release with applied park air? Wonky actuator or wedge? The operation is the sum of all the parts…

Those cans are going to apply something like 2,800 lbs of force to retract against the springs that probably push back with 1400 lbs (back of hand calculation based on can diaphragm and 120 psi tank air, and ~60 psi balancing the springs where you start to feel the parking brakes drag) so net of like 1400 lbs pulling on those wedges.... would have to be pretty hung up but maybe.

My wedges and adjusters were really rusted and the parking brake cans just muscled their way past it with ZERO complaints. So much power......

Although there's no linkage to allow the can to actually *pull* the wedge so I guess it could just get hung up and then drum brakes have this nasty property of being self-energizing....... hhhhmmm yeah possible.

 

[dwlindsey]

I guess another possibility is that the spring brakes did not fully release with applied park air? Wonky actuator or wedge? The operation is the sum of all the parts…

I thought of that as well, but when I caged the canister, the shoe was TIGHT to the drum and the actuator was extended a lot, not where I believe I put it yesterday. So I don't think it is a park air issue.

At this point, I have no idea what happened. I suppose it's possible that I somehow adjusted it wrong . . . but I checked and re-checked with a feeler gauge . . .