questions regarding air over hydraulic brakes on M39/809 trucks

[steelcaptain]

is there any description available how the air over hydraulic system functions? I assume the master cylinder provides hydraulic pressure to the air assist cylinder which itself then adds pressure to push the fluid to the wheel cylinders but how does the air assist do this? I also am understanding that without the air assist connected, the brakes do still work albeit requiring much more force on the brake pedal for the brakes to slow the truck?

I am also aware that with a blocked vent in the front service brake gladhand, that the brakes will not release when applied but is there any other part of the system which would cause the same condition if it fails (ie. in the air assist valve)?

I ask all this as I am considering an M813 which has this problem at present but the truck is 300miles from where I live and I wish to be able to make it drivable where it sits even if this requires disconnecting the air assist from the system?

 

[Mullaney]

You wouldn't want to drive a vehicle with air over hydraulic that wasn't functional. You would stand on the brake peddle with both feet and pull on the steering wheel and slow down. Actually stopping where you want to stop on the other hand - not very reliable without the brake booster doing it's job. If you want to test it with your pickup truck, pull the vacuum line (and plug it) on your power brakes. Then on a flat surface with nothing in your way, try stopping at about ten miles an hour... It isn't pleasant and in an emergency stop it would make your heart pound for sure. The M813 does have a manual transmission. That with plenty of notice that you need to slow down could do as much as standing on the brake peddle.

Just tossing ideas out now...
Has the truck been parked for a while?
Do you know if if makes air? IE: Is the compressor good and are the tanks good?
Any idea of the condition of the brakes when parked?
Do you have access to another 5-Ton that you could Tow Bar it home with - to work more conveniently?
Is it Interstate all the way home or could you go slower on back roads?