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1979 MR685S - brakes on rear dragging


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I bought a Mack tanker fire truck and had it shipped to me. When they attempted to load it, one of the rear axles drug in the gravel. The fire chief said it didn't do that when they parked it. It sat for about 6 weeks before I got it. It wasn't locked up and they were able to load it on the step deck. I drove it off the step deck and you could definitely tell it wasn't right. I had to drive it about 300 yards to park it which was no problem. Any ideas? I was going to check the slack adjusters but is there anything else I should be looking at? Thanks

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also you can mark the push rod then release the brake and see how much rod travel you have.(should be 1 1/2" not too exceed 2") if properly adjusted

once released you can unpin the clevis (make sure wheels are chocked) and back off the slack adjuster to get room for your hand to grab the slack adjuster arm then stroke the adjuster and look for freedom of movement in the drum of shoes, rollers and pivot pins (anchors)  and the S cam. repeat on the other side . then readjust the brakes  turn adjuster until firm (shoes should be in contact with drum surface) then loosen 1/2 turn. 

You could also have a faulty air valve remove the air hose from the air can ( the hose closest to the push rod) no air should be present at that port with the brakes released and the foot valve released (check the foot valve tor free operation, the cross pin can seize in the housing and not fully release) if air is coming out from the air can port you could have a bad center section in a maxi can,      

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  • 5 months later...

Update: I couldn't work on the truck through the winter but finally have done a bit of checking and followed your advice gearhead. I unpinned the clevis and backed off the adjusters. Manually moving the adjuster back and forth is relatively easy and I can see the brake shoes moving. I think that's ok. I was about to remove the airline closest to the push rod but noticed that the foot valve is really hard to push. I'm wondering if it's the air valve. Any advice?

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  • 1 month later...

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