Grind

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.