Grind

Update: Removing the Rotochamber and blocking that line off was the ultimate fix, since the front air over hydraulic brakes weren't needed anyway. Still have one leaky valve, impacts the secondary system only as the primary is charging to 120 but the secondary bleeds off to ~90 as the truck idles for awhile. I believe it's a relay valve and when summer hits I'll remove it and see if I can get a rebuild kit for it and get it dialed in. Thanks again for all the helpful feedback. Now just need some damn snow so I can feel better about all the money I sunk into this project.

You're not wrong. I found this forum looking for info on Google as it consistently shows up in the top results for anything Mack + specific query. I ended up buying a couple manuals, one generic one that handles Mack air brake systems and a bigger one for the entire truck series that also covers the topic. Trying to track the lines to match the manuals didn't really accomplish anything, I'm questioning if they might have just left some old lines strapped to the chasis and ran new ones at some point. Capping off the line to the Rotochamber resulted in a lack of air pressure to actuate the new rear brake job so I'm currently spraying the bolts with PB Buster and will remove it. There's something else mounted behind it (rectangle about 2" deep, no idea here) against the chasis that has air lines running into it so I suspect somewhere in there is the culprit. I'll update the thread with results as I've seen elsewhere that people appreciate that but I'll try to keep the questions to a minimum. Also happy to shoot photos of the air brake system manual pages and upload them if that would be useful to anyone, if it doesn't violate forum rules. Thanks for all the help.

Yeah, the 1979 R350 was off the Craigslist ad I bought it from, that guy didn't know any more than me lol. I found a manual for sale on ebay for the DM685 that I think I'll pick up but I wanted to make sure I was on the right path first. Flat flange on the Axles. Thanks for taking your time to share knowledge, appreciated.

I don't know if it's bad form or not to respond to my own year old post but after 6 months of trying to get a mechanic out here to help, finally here. We capped off the alleged Rotopacker, replaced all the shoes/bearings/seals as they were glued to the hubs pretty good but still having an issue with the air system. Been searching online for manuals, not much luck. Here's the vin and door tag, thanks for responding. Edit: Your other post asked about rear brakes, both axles are running air brakes.

Looks like it's connected to a master cylinder. Cousin says the front brakes (that are no longer attached) were air over hydraulic so it probably powers those? No long threaded rod visible but maybe it's inside the master cylinder? One hydraulic break line comes out of the MC, goes up the chassis towards cab. Out the top of the MC is what looks like a 3/8" air line, disappears into the cab firewall, haven't tracked it down further. Photos attached. Red air valve, see attached photo. Thanks for your time and knowledge, much appreciated.

Found this forum because literally all roads lead here when you start researching Mack trucks. Hopefully one of you wise ones can tell me what I'm looking at...and more importantly, will I die if try to fix it. 1979 R350 plow truck with spreader. Bought it, drove it home (~50 miles), plowed a couple times, then it sat over summer. When I went to move it out of the way, the low air alarm wouldn't stop buzzing and my brakes were locked on. My cousin has driven Mack trucks (all trucks) so he came out to help me trouble shoot it and we tracked down the air leak to the pictured unit. Mounted on the passenger side frame behind the cab, it has one supply line coming into it and then one line going back up into the cab (didn't get further than this). When we clamped the supply line with vice grips, it built pressure up to 120psi no problem but the brakes wouldn't release fully (per my cousin...driving it to get it out of the driveway, I didn't notice it like I did when I tried to move it this summer but I'll default to his greater experience as I'm a truck neophyte). It's dark now but I can get any other pictures necessary for ID tomorrow when it's light. Thanks in advance for any help, been reading the forum for awhile and I'm blown away by the collective knowledge base here.