Brakes not releasing

[Greasy]

Over the last week or so, I’ve had a brake issue with my M-548 wrecker. After depressing the brake pedal the brakes will not release again. The harder you depress the pedal, the harder the brakes apply. The brakes will only release if you crack a wheel cylinder bleeder at a wheel on any axle (I’ve checked all 3 axles). Is there a common flex line between all axles, air pack issue? My wrecker has a manual maxi lock valve and has been adjusted out (to eliminate the valve being an issue). I’ve not messed with an air pack much to know if this can be an issue. Currently this truck is parked at a airport museum, and can be tested without traffic being an issue. Any assistance would be greatly appreciated.

 

[G744]

Possibles in order of probability:

1...Brakes adjusted too tight
2...Master cylinder internal valve sticking.
3...Wheel cylinder internal corrosion
3...Airpak faulty (check to see if problem still exists with no air in system)
4...Shoe return springs bad or shoe pivots corroded.

Could also be gunk in the system from someone mixing DOT3 with DOT5 fluid.

DG

 

[Greasy]

Possibles in order of probability:

1...Brakes adjusted too tight
2...Master cylinder internal valve sticking.
3...Wheel cylinder internal corrosion
3...Airpak faulty (check to see if problem still exists with no air in system)
4...Shoe return springs bad or shoe pivots corroded.

Could also be gunk in the system from someone mixing DOT3 with DOT5 fluid.

DG

Thank you DG,

I will try the brakes with the air system drained and look into a master cylinder rebuild kit. The brake adjustment seem OK (by sound after I adjusted them). My main question would be what could cause ALL the wheels to lock up together when applied.

 

[US6x4]

I've heard of this happening when the system wasn't able to release pressure via the vent line that exhausts through the front rh side glad hand. The issue i heard of had the hard vent line pinched somewhere near the wheel well. That's something to check on...

 

[fasttruck]

I had a similar situation with a M35A2 which was eventually remedied by replacing the airpack which was not reacting to the release of the brake pedal. truck had DOT 5 in it.

 

[Mullaney]

Over the last week or so, I’ve had a brake issue with my M-548 wrecker. After depressing the brake pedal the brakes will not release again. The harder you depress the pedal, the harder the brakes apply. The brakes will only release if you crack a wheel cylinder bleeder at a wheel on any axle (I’ve checked all 3 axles). Is there a common flex line between all axles, air pack issue? My wrecker has a manual maxi lock valve and has been adjusted out (to eliminate the valve being an issue). I’ve not messed with an air pack much to know if this can be an issue. Currently this truck is parked at a airport museum, and can be tested without traffic being an issue. Any assistance would be greatly appreciated.

.
You mentioned a "manual maxi lock valve". We used to use a "BrakeLok" at one point in time that would lock the fluid in the lines (apply the brake as if your foot was on the peddle). The BrakeLok had a nylon tip that rode on a cam (pushed in / released the brake) to release the brakes.

You might look if your "maxi lock" works the same way. If it does, gently prying on that nylon button will release the brakes. And, if that works you need a new button because of wear or age.


EDIT: I added a picture from MICO, Inc. They have something very similar and the "button" is definitely in this picture. You can also see the "cam" that releases and causes the brakes to stay applied as mentioned above.





EDIT #2: I found the instructions that specifically mention the "wear button" and how it may be causing your specific problem. There is a four page PDF attached here just in case you need the information.



I would love to know if that solves your particular problem...

Tim

 

[BEASTMASTER]

i had an issue like that during a 4th of july parade once, replaced the master cylinder and had no more trouble

 

[Greasy]

.
You mentioned a "manual maxi lock valve". We used to use a "BrakeLok" at one point in time that would lock the fluid in the lines (apply the brake as if your foot was on the peddle). The BrakeLok had a nylon tip that rode on a cam (pushed in / released the brake) to release the brakes.

You might look if your "maxi lock" works the same way. If it does, gently prying on that nylon button will release the brakes. And, if that works you need a new button because of wear or age.


EDIT: I added a picture from MICO, Inc. They have something very similar and the "button" is definitely in this picture. You can also see the "cam" that releases and causes the brakes to stay applied as mentioned above.


View attachment 830505


EDIT #2: I found the instructions that specifically mention the "wear button" and how it may be causing your specific problem. There is a four page PDF attached here just in case you need the information.



I would love to know if that solves your particular problem...

Tim

Thank you Tim,

You provided great information! This weekend I will be out at the museum and will look at the button on the brake lock, as your diagram shows verify if the button has excessive wear, if it moves freely and confirm the manufacturer name.

While reading the PDF, I did remember working on an old Chevy truck with the "hydra boost". Someone before me had replaced the master cylinder that had an internal piston depth difference, causing the rod to push the brake piston in slightly applying the brakes. Once I modified the push rod, problem solved. I will verify this weekend if I think the master cylinder push rod is "free" in the bore.

Thank you to everyone with the advise, this is exactly why I love this site! Along with feeding my green vehicle addiction.....

 

[NY Tom]

Late to the party but I recall there being a small hole in the bottom of the master that the return fluid comes back into?

I have read around here that this hole can become blocked and prevent the release.

No one else mentioned that in this thread. Someone more in the know might see this post and chime in with better detail.

 

[Mullaney]

Thank you Tim,

You provided great information! This weekend I will be out at the museum and will look at the button on the brake lock, as your diagram shows verify if the button has excessive wear, if it moves freely and confirm the manufacturer name.

While reading the PDF, I did remember working on an old Chevy truck with the "hydra boost". Someone before me had replaced the master cylinder that had an internal piston depth difference, causing the rod to push the brake piston in slightly applying the brakes. Once I modified the push rod, problem solved. I will verify this weekend if I think the master cylinder push rod is "free" in the bore.

Thank you to everyone with the advise, this is exactly why I love this site! Along with feeding my green vehicle addiction.....

.
Hi Greasy ,

I was rummaging for something else and stumbled across this thread.
Just curious what the outcome was with the "sticking brakes / brakes that won't release" problem on your M-548?

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Doubt my latest problem has anything to do with it, but I have short story:

I have a fair weather M-1088 where the shoes stick to drums if it rains and the truck doesn't go for a trip around the block. It seems that the angle of the truck - tipped (horizontal ) left down hill and that shoe sticks to the drum. Turn the truck half way around and the right downhill shoe sticks. It only seems to happen to the center axle. A sprinkle of rain won't do it, but a gully washer will do it in about 48 hours almost every time...