379 Brake Problems

Hi all, I'm new to the forum and looking for some experienced input on some continuing brake issues on our Peterbilt.
Info on the truck: (I'm new to wrenching on big trucks so forgive me if I get some terminology wrong)
It's an '05 PB 379 (Wireline Truck), this truck was based up in Alberta, CA and when they shut our base down it managed to make it's way down here to Houston, TX. I'm in the process of getting my CDL to drive this truck as they've laid off all the licensed drivers, so they have given this truck to me and told me to get it going like it's supposed to and keep the maintenance up on it.

I've gotten mixed stories on this truck, guy's that have come down here for training from CA told me that they think this was one of the trucks in our fleet that got rolled down an embankment and ruined the cab and others are not sure. The wireline operators control room on the back was leaking water like a siv and I had to tear the whole inside out and replace it. The biggest issue among other various things are the brakes. About 2 years ago we had a mechanic come out to fix the brakes and apparently had no idea what he was doing because he got under the truck and jacked with the brakes, got in threw it it reverse and away he went (right over our fence). It cost that company several thousand (not including truck damage), the brakes work good when you put it to the floor, but that's really the only time they work. When I put the pedal down it sticks and have to reach down and pull it back up by hand, the also causes an issue with the brake sensor on the idle-up to run our PTO and will not let it go. I have to pump the pedal several times and then give it a good jerk up to get it of the sensor.

I talked to a family member who works on trucks and said to look at the foot valve, my question is where is the foot valve (obviously by the pedal), but what does it look like? I decided to join the forum because this is not the only issue and will probably need some more assistance in the future, but the brakes are #1 on this list. Any and all info that can be provided is GREATLY appreciated and I'm open minded so shoot me ideas and whatever other kind of info that can benefit me on the build.

Thanks, Bradley

 

So are you saying that I'd be better off just replacing it? I think that would be the best option to make sure that it's functioning properly, is it inside the cab under the pedal? This thing came out of the great white north and the whole chassis has a coating of rust on it so I'm guessing it got plugged up from road salt and harsh conditions. I forgot to mention that this truck sat untouched for 6 months and the tanks were full of moisture.