In all the years I've been around buses I have only had to replace one 740 due to failure and that was due to an incompetent driver who damaged the pan, had someone "patch" it for him, and forgot to refill the fluid to the proper level before heading off on a 200 mile trip.
He made it 150 mile before it locked up in drive!
We could start the bus and run it in neutral, but if we put it in gear it'd kill the engine.
I had a friend come haul it in to our shop. (I was on the road too with a different team, and actually came across "dumb driver" on the side of the road trying to pour 5 gallons of Co-op trans fluid in after it locked up!)
When the tow truck driver called me and asked "hey does this thing run at all?"
I told him "Sure, but it will die as soon as you put it in gear!"
He said "well I was hoping I could build air pressure, release the brakes, & let it roll backwards down the hill and back it onto the side road behind it where I'd be able to hook up and head back to the interstate w/o having to turn around after hooking up".
I told him go for it, it's already toasted!
Much to his and our surprise once he was barely rolling backwards he was able to stick it in reverse and it actually "broke loose and" pulled itself back around the corner to where he needed it to! (good thing too as he couldn't get the needed momentum just coasting)
Now that said I have replaced several multi-mega-mile 740's as preventive measures before they failed, but as said the dang things are damn near indestructable!
Now on the other hand in our coaches that have the B500 the 3 older models (2 '95's & '97) we have all had transmissions put in them not long before we bought them @ around 300,000 miles.
Now jump to the (2) 2005 models we have. One of them lost the trans @ 275,000 and the other is starting to act up @ 330,000.
Which is about par for what everyone has told us. (Even Sammy our truly beloved departed B500 guru had told me to expect to replace the B500's around 300,000!)
So there ya have it.