JoeH

Are you saying you have air leaks? Can you clarify what you mean here? With truck fully assembled you need to run through a proper Air System PreTrip Inspection! This will find any air leaks you have that could be causing your issue! Maybe I'm not following the thread well enough, but I don't get the impression that air leaks were checked for "by the book." Here's a YouTube video on how to run the air system pre trip inspection.

IIRC on this style clutch you take out the lock tab, push the clutch pedal in, then spin the flywheel to thread the clutch tighter? Then reinstall the lock tab and you're good to go... I may be a weee bit off though...

Looks to me like 4034836, but idk. Probably a 30 or more year old clutch, don't think my dad ever put a clutch in it, and we've had the truck since I was a boy 25+ years ago.

Agreed that looks like a truck for Europe. No thanks on that electric monstrosity, I'd hold out for an Edison Motors repower of my old Macks.

Remembered I have a 237hp motor from a 1976 U685T sitting here with the transmission off. Has a Dana Spicer clutch on the flywheel. Here you go:

If the brakes aren't working I'd bring joey to the truck lol!

Some truck manufacturers install a pressure gauge to tell you how much brake pressure you're applying. You can put a gauge on your system so you can see if you're getting good pressure through the foot valve. I suppose it's possible you hurt the foot valve, but I doubt it. I assume the truck has self adjusters? I think you need to be under the truck while someone else works the pedal to see if all the slack adjusters are functioning properly. Maybe you ripped one of them up internally. Does the truck pull to one side or the other when stopping? The brakes would be working better on the side it pulls towards. Have you run through the pre trip air system check to listen for air leaks? Maybe you ruptured an air chamber.

Do you have access to a helper? You need to have someone in the cab working the brake while you are under the truck. With wheels chocked of course!

You could also have broken friction material off the shoes, or cracked a brake drum. When the brake drums get worn out and thinner they're easier to blow apart. But I'd expect that carnage would be obvious.

I'm wondering about S cams going over-center. If any brakes were close to being out of adjustment then a real hard brake application can push them far enough that the tips of the S go passed the rollers. On all my trucks I release my brakes, put a zip tie on the pushrod right against the air chamber, then apply the parking/trolley brakes. Measure the travel of the zip tie from the reference face of the air chamber, more than 1-3/4" travel is a red flag, and that wheel needs to be adjusted. That's my redneck check my brakes myself method when I don't have a second person to work the pedal. It is also possible to bend the bracket the brake chamber bolts to. Its a replaceable plate, you have to pull S Cam and then it's 4 bolts around the tube the S Cam goes through.

I think I read on here a few years ago that you should check your coolant with a voltmeter to check it's health. I forget what settings to run and what number range to look for, but the engine coolant i tested checked out well. I guess the stuff goes bad and will cause electrolysis, which is basically what the voltmeter is testing for.

https://youtube.com/shorts/l5_uUaoLTmg?si=c6usxvkAkBoe3VcC

Personally I would just get the right clutch for your existing truck. Chances are that'll be the last clutch that truck ever needs, unless you're going to be putting 1,000 hours or more on it per year.

Turns out this donor MR was a municipal dump truck out of New York. The frame rusted and a driver drove the raised dump body into a bridge overpass, so it's been sitting at Coopersburg Kenworth with about 14,000 miles on it since about 2014! Can wiring looks great, and with so few miles the transmission and engine are barely broken in!

Having a complete donor powertrain with all electronics (minus the rears which are easy to get) is sweet. Hopefully I never have to part it out to keep the MR running, but I'll do what i need to.

Yup!

Found a cab on Friday and sealed the deal on it. Insurance appraiser is going through a family emergency at the moment, which may slow our process dont a bit but we work to live, not live to work, so we hope all is well with him. Dad and I and everyone we've talked to/showed the truck to are pretty well convinced this truck is definitely fixable, we just may have to push a little, which goes well enough with my confrontational personality, so it should be heading over to Bergeys in the next day or so to get the ball rolling. The cab we found is through COOPSKW where my dad bought our first Mack (our '79 r686st, which is still running most days because this MR is taking a break...) in the mid 1980's. Cab is a whole front cutoff of a 2000 MR688 with a running 300hp right side EECU ETECH with an Allison HT740 cable shift transmission. Cutoff should be going to Bergeys for them to swap all the electronics and wiring from my truck into the replacement cab. I'll be sure to make sure they try to keep all the wiring and electronics from the 2000 intact so I can transplant them into some other truck some day if I take a notion to it.

He's working on this dog. Its a short wheelbase, the driveshaft will definitely need to be out of the way.

RD models ran 2 solenoids, not including the one on the starter itself. One on the firewall, one under the dash. The one under the dash runs power to all accessories, gauges, etc. The one on the firewall runs the signal to the starter. Being a 1999, I'll wager you have a Vmac III ETECH engine. If you don't have power to the EECU then you won't be able to start the engine, even if you can get it to crank. Check your fuse diagram for the EECU power fuse. It should have power to one side when the key is in the on position. If not then you need to fix your power fault. I'd guess you have an accessory relay gone bad, given your lights arent working.

Usually takes us a day or two to get it out, and another 3 days of rocking the transmission back and forth and up and down to finally get the input shaft into the pilot bearing. 😱 When we pulled the allison transmission out of our MR688 2 years ago we had the whole thing out and back in in one day! That MR is the easiest thing to work on. Our 1980 DM is about the hardest thing to work on. Don't recall how hard the clutch was on our DM, but it might not be too bad. Our R models we usually stick a heavy wall pipe through the windows and hang the pipe from the roof with a rope going back and forth a bunch of times. Gives us a beam crane for our chain hoist inside the cab to hang the transmission from.

I'm really itching to flag a manager down at home Depot when I'm in self checkout to tell them I expect an employee discount if I'm gonna be doing their job for them.

We usually take a fuel tank off the truck so we can get the transmission out from underneath, but if all you're doing is a clutch then you may not need to.

The Mack triple shaft transmissions sit like a V , it will want to fall over on a trans jack, so a cradle is a must have! Do yourself a favor and replace the rear cab mount bushings, we just did on our '79 R model and gained more than an inch of height back. Makes a lot of difference when you're working in tight spaces. Access to a helper is strongly recommended, there's a number of things that really need 2 people.

Inquiring minds want to know how you're making out!

https://maps.app.goo.gl/ea5bGe6WJHoKPRKL6 A&R industrial is near you, no idea what their reputation is. If you're confident you can do the repair yourself then get it apart, take your old clutch to them and they can order you a new one based on the parts you show them. Assuming they're the friendly and knowledgeable type. If you aren't confident then you could just have them do the repair.