Grind

5 hours ago, 67RModel said:

Now that I look at the first picture of the chamber more closely I see it is just behind the cab on the passenger side next to the exhaust. With the amount of wheelbase the truck has its hard to believe it is part of the braking system. What is it doing? what does the end opposite of where the airline is going in look like? Does it have a long threaded rod coming out of it and if so what is it connected to?

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.

5 hours ago, 67RModel said:

That is an old Rotochamber......YUCKKKKK! Its hard to see from the pictures supplied, but is definitely a rotochamber of some sort. The ones I have seen are just single acting service brake chambers like a steer axle chamber. No spring brakes. My B81 had them and just had a brake drum on the driveline for parking. The other ones I have seen were on a DM800 but also had single acting, spring, pull type Maxibrake chambers opposite facing of the rotochamber connected to the slack adjuster with all thread or chains. if there was a loss of air the spring maxibrakes would pull on the slack adjuster.....Very queer setup but was a solution before 30/30s and similar were common or perfected. I don't know if the truck pictured has Maxibrake, pull type chambers on it but what I am seeing in the pictures is just a single acting rotochamber for service brakes. Very obsolete an insanely expensive to replace if you can find them. Like $600+ each. I think they are serviceable and there is just a regular rubber diaphragm in them if they are leaking internally. Very hard to believe these are on a 1979 but hey anything is possible. 

Does the truck have a yellow air valve on the dash for parking brakes? If not then I would guess the brake lining is stuck to the drums. Sledge hammer is in order.....

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.