xm817 brake issue

I've had my 5 ton for quite a while. Brakes have always been unpredictable,but worked. I use my truck to haul stone from my stone pit for my driveway care. this year when i got it out to use,naturally brake fluid was low,so i filled it up. started out working ok, then it got to where you can not push the pedal at all is there any help for me?

Check the glad hands on the front of the truck. The one on the passenger side needs to be vented.

I checked gladhand it was plugged. thought that was going to do it until i took it for a drive it might be a little better but still not safe. part of the timei can hear sound of air coming, then vent when i let off the pedal....

The brake booster needs to be replaced most likely

also check the air check valves that run along the frame (under the driver seat area approximately)

thank you i'll check them.how many are there? maybe that's why they sometimes work.