Brake Issue with '72 M818

[GFrost Roadkill]

Looking for help. All six hubs lock up with brakes and stay locked. First foot depression is soft then locks up hard and engaged. Dropping air does not release, only letting sit for awhile does. Can be driving down the road, not touch brakes at all, and brake lights come on and they lock up tight. Any ideas? Thanks.

 

[gimpyrobb]

Probably need to go through most of your brake system.

To me it sounds like a pad or two is rubbing making heat which expands the fluid, effectively putting more pressure on the brakes cause your air pack and master cyl need rebuilt.

 

[73m819]

Is the brake pedal return spring broke, with the pedal a bit forward, the fluid can not return to the M/C and will lock things up, this plus a bit of pdeal viberation pumping the brakes (even a little bit) will pressureize the system and lock things up on it's own,

I am going this route because you say even if you dump the air, the brakes STAY LOCKED

 

[Ferroequinologist]

I agree that it is most likely a pedal adjustment issue or an airpack in need of rebuild.

 

[GFrost Roadkill]

That is an interesting thought but pretty tough to trouble shoot and diagnose. I have torn down all 6 hubs for rebuilt and cleaned brake mechs and they don't seem to be rubbing abnormally. They are either completely free or locked, nothing in between.

 

[GFrost Roadkill]

If I manually pull the peddle back up they should unlock if a peddle return/adjustment issue, correct?

 

[gimpyrobb]

Unless the air-pack needs work too, they should.

 

[GFrost Roadkill]

With the airpack theory, wouldn't they just feel really rigid vs. locking up?

 

[BigWill1985]

I had similar problems on my 1978 Chevy C-50 dump truck. Problem turned out to be condensation in the master cylinder from sitting. I pulled the master cylinder, honed the cylinder, and ran a drill bit through the ports (significant amount of rust was in the smaller ports). I concluded that the return port was blocked, which maintained the brakes slightly applied, which made the fluid boil and locked up the brakes. After the rebuild, I bled the system and all has been working perfect since.

~Will Courtier~

 

[73m819]

If I manually pull the peddle back up they should unlock if a peddle return/adjustment issue, correct?

Pedal return yes, adjustment no, the pedal will hit the floor board before the piston is all the way back

 

[augiedoggy]

How about cracking a bleeder valve when locked, there is a bleeder on the airpack too.
just a thought, good luck.

 

[NDT]

Check to be sure the RF glad hand cover has a little hole in it.

 

[Tow4]

I had to adjust the brake pedal on my M818 before I drove it home. You need .5 to 1" play at the pedal if I remember right. Check the TM to be sure. If the pedal is too tight the vibration of it running or driving down the road will cause the brakes to come on or not release.

 

[o1951]

Master cylinder bleed port is either plugged per post #9, or pedal improperly adjusted. Pedal must allow piston to uncover bleed port or what are experiencing happens.

 

[OshkoshEquipment]

see post 12. If the right hand front glad hand is blocked completely or there has been a shut off valve added the pressure builds up in this line and applies the brakes like it is being towed like a trailer.