Tracked Down An Air Leak, Need Help ID'ing Part

[Grind]

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.

[tjc transport]

welcome to BMT!!!  wise ones.... you must have been reading Joey Mack talking about himself.. again.

will you die trying to fix it? probably not. 

but make sure you have a very large supply of band aids and mercurochrome to staunch the bleeding from all the cuts you will inflict on your arms and hands.  🤣

please be patient, and someone much wiser than me at fixin these beauties will be along t help you.

(i dont fix them, i drive and break them)

 

[Joey Mack]

Now I have a complex...  Thanks TJC..  here's one for ya,   I have no clue what that part is...  So there.....  😘

[67RModel]

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.....

[67RModel]

https://www.ebay.com/itm/305293929948?epid=6020591473&hash=item4714efaddc:g:XlsAAOSw07xlaIKz&amdata=enc%3AAQAIAAAA4FfL4slc%2FfeTIfN4OvvbVT%2F2%2FkdstS6lyDeiODSEWB%2BCRCpdK4uzo9seCgwqhDwdS5glAzMfVBlXnvI8UbyBaOfBIcr4F8Eo84qPsvIon5ReFzpMk%2Bb4IPYef1LRPzz5cT5QXx%2BRhoJONJYN62eX8Mgqb8eNEsg%2FgMkAoSVvjHD4mLaxfov2YlVDh9A3gZIEbRdilTXOi%2Fga1rZD2aeWY0%2BGZGQXQ5wz8X06NII0mnTB06OEZDUjiZ%2FU7EwQQfhL48NJJb8HOV4S7%2BpLuQZqP6M51f%2BqAm1Z7%2FcP2iq4qti0|tkp%3ABFBMwobWzo9j

Try not to vomit when you see the price.....

[67RModel]

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?

[fjh]

1 hour ago, Joey Mack said:

Now I have a complex...  Thanks TJC..  here's one for ya,   I have no clue what that part is...  So there.....  😘

Yup Im at a total loss as to that! It has what looks to be another valve where the rod should be on a roto chamber pot!🙃

[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.

[67RModel]

57 minutes ago, Grind said:

Red air valve, see attached photo.

Thanks for your time and knowledge,  much appreciated.

 

Ok WOW. WTF even is that thing? Are you able to trace the lines going into and out of it? Where does the cloth braided line feeding it come from and where does the plastic line coming out of the brass 90 elbow go to? It also looks like it has a grease fitting on the end of it????

Also its pretty strange it is a straight truck and has a red button only but that could just be the old yellow diamond got brittle and and broke. The the old red/yellow buttons were removable off the valve with a small roll pin. Maybe they just threw that red button on the valve that should be there. Does the truck have hookups for an air brake trailer? Notice it says "trailer air supply".

[JoeH]

I have 2 wheel loaders that use air over hydraulic brakes. The rotochamber applies the rod movement to operate the hydraulic master cylinder. If that one only operates the front brakes and the front brakes are already disconnected then you can unplug the air line from the rotochamber and cap it. Before doing this, verify that this rotochamber/master cylinder doesn't operate the rear brakes as well. If you plan on using this rig on the road then I'd say you need to fix the front brake system.  If you're just using it on your own property then the safety of your own property is your call.

You should be able to convert that rotochamber over to a Type 24 or Type 30 air chamber instead.  

[JoeH]

Type 24's and Type 30's are far cheaper and more common. Rotochambers are basically just old style air chambers.

[JoeH]

The type # indicates how many square inches of surface area pressure the brake chamber has. So a type 24 chamber has 24 square inches of surface area to compound the applied pressure. If you're doing a soft stop at 30 psi then 30 psi x 24 sq in is 720 lbs of force applied on the pushrod. If you're doing a hard panic stop and you're applying 100 psi then you're applying 2,400 lbs of force on the pushrod.

If you're running type 30 chambers then those braking forces would be 900/3,000 lbs of force of the pushrod respectively.

That rotochamber looks pretty big, it might be a type 36.  You can measure the chamber diameter and run the math to figure out what size it is.

3.14 x radius squared should give you the size of the chamber so you know what size you'd need to replace it.

[fjh]

20 hours ago, Grind said:

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.

All makes sense now! Ford used to use this setup on the Louisville back in the day  on front Disk app! never seen it on a Mack!

[67RModel]

Wow. Air over hydraulic brakes. I have never seen nor heard of that on a Mack. What would be the benefit of doing this? I mean you already have an air system and are using rotochambers to actuate. Why would one decide to actuate a hydraulic master cylinder rather than a slack adjuster? I'm curious.

[Vladislav]

I have that similar setup on my 6x6 DMM chassis. Or more correctly to say, restovers of it.

Exactly as JoeH explained. DMM has hydraulic brakes on the front axle. And that thingy (that old airbrake chamber) used to activate the master cylinder for sypplying juice to the calipers.

[Vladislav]

3 hours ago, 67RModel said:

Wow. Air over hydraulic brakes. I have never seen nor heard of that on a Mack. What would be the benefit of doing this? I mean you already have an air system and are using rotochambers to actuate. Why would one decide to actuate a hydraulic master cylinder rather than a slack adjuster? I'm curious.

In my case it looked like the setup was determined by the vendor of the front axle. From pics on Truckpaper I got figured some DMM's had air brakes at the front.

[mechohaulic]

title calls it an R-350. thought was an R-400 ; would  put it in the hyd brake class ??. picture  looks to be a twin -screw.  red button may not be a miss place if it is set up for trailer towing??  interesting to find out what rear drive brakes are ;; hyd/air. would help explain the conversion roto chamber for air to trailer brakes.  all goes well , BMT will have follow ups and great to have complete door/ frame serial number.

[mechohaulic]

picture #2 showing roto chamber also show a hefty driveshaft , TRANSMISSION (not falling for that trans stuff again), not having a parking brake drum on output shaft ,what appears to be tri shaft  maxi TRANSMISSION . picture showing the red parking brake button has partial decal on heater box cover looks to say maxidyne diesel engine. vin number will explain  much

[Grind]

On 12/19/2023 at 5:05 PM, mechohaulic said:

picture #2 showing roto chamber also show a hefty driveshaft , TRANSMISSION (not falling for that trans stuff again), not having a parking brake drum on output shaft ,what appears to be tri shaft  maxi TRANSMISSION . picture showing the red parking brake button has partial decal on heater box cover looks to say maxidyne diesel engine. vin number will explain  much

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.

 

Edited December 21 by Grind

[mechohaulic]

updates ALWAYS appreciated ; thanks

the DM685 S makes a lot more sense. the R350 didn't.  '77vs '79. as with any restoration ;; in time it will all come together.  DM=Dump/Mixer ==685 -maxidyne engine/transmission == S lighter chassis specs. updates including pictures makes answering questions also easier.  S in model would indicate possible 34/44 rears . 55 would be SX. axles have cone caps or flat flange ?

[terry]

2 hours ago, mechohaulic said:

updates ALWAYS appreciated ; thanks

the DM685 S makes a lot more sense. the R350 didn't.  '77vs '79. as with any restoration ;; in time it will all come together.  DM=Dump/Mixer ==685 -maxidyne engine/transmission == S lighter chassis specs. updates including pictures makes answering questions also easier.  S in model would indicate possible 34/44 rears . 55 would be SX. axles have cone caps or flat flange ?

S means six wheel vehicle.  L means lightweight components  terry

[Grind]

2 hours ago, mechohaulic said:

updates ALWAYS appreciated ; thanks

the DM685 S makes a lot more sense. the R350 didn't.  '77vs '79. as with any restoration ;; in time it will all come together.  DM=Dump/Mixer ==685 -maxidyne engine/transmission == S lighter chassis specs. updates including pictures makes answering questions also easier.  S in model would indicate possible 34/44 rears . 55 would be SX. axles have cone caps or flat flange ?

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.