Virtual400 Posted November 28, 2016 Share Posted November 28, 2016 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 Quote Link to comment https://www.bigmacktrucks.com/topic/47813-1979-mr685s-brakes-on-rear-dragging/ Share on other sites More sharing options...
Hobert62 Posted November 29, 2016 Share Posted November 29, 2016 Take the rubber plug out the end of the maxi brake and see if the spring is ok. I've had them break and get turned sideways keeping the break from fully releasing. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/47813-1979-mr685s-brakes-on-rear-dragging/#findComment-353407 Share on other sites More sharing options...
gearhead204 Posted December 6, 2016 Share Posted December 6, 2016 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, Quote Link to comment https://www.bigmacktrucks.com/topic/47813-1979-mr685s-brakes-on-rear-dragging/#findComment-354112 Share on other sites More sharing options...
Virtual400 Posted May 15, 2017 Author Share Posted May 15, 2017 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? Quote Link to comment https://www.bigmacktrucks.com/topic/47813-1979-mr685s-brakes-on-rear-dragging/#findComment-369833 Share on other sites More sharing options...
Underdog Posted May 15, 2017 Share Posted May 15, 2017 Foot valve is siezed. There is a steel plunger in the aluminum pedal base that could be stuck, preventing the brakes from releasing. Take it all apart and clean it and grease it.Sent from my XT1254 using Tapatalk Quote Link to comment https://www.bigmacktrucks.com/topic/47813-1979-mr685s-brakes-on-rear-dragging/#findComment-369837 Share on other sites More sharing options...
Virtual400 Posted July 8, 2017 Author Share Posted July 8, 2017 Underdog - you were right. The foot pedal was binding. Pulled it apart, cleaned it up and greased it. It's working now. I need to adjust the brakes and I think it's good to go. Thanks. 2 Quote Link to comment https://www.bigmacktrucks.com/topic/47813-1979-mr685s-brakes-on-rear-dragging/#findComment-374450 Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.