I would first check inside the cab for material under the brake pedals, these machines with the pedals on the floor have a history keeping the brakes applied if the floor is not kept cleaned up. While working the workers boots push mud, dirt,stones under the lower edge of the pedal causing the brake pedal not to release fully and can lead to your problem. First alway's is safety with you and workers around you, this machine is about 4 yard size and rear brakes alone will not stop it very good with a full bucket and will trash what is left of you rear brakes. Once the pedals in the cab are checked, check main air line to the booster for air flow( engine off, air system charged) crack the line and lisen for air flow to the chamber, if there is alot of air pressure there the foot valve is leaking by inside or the pedal is still not releasing fully. Next remove the bleeder screws from the rt/lt calipers or crack the line to allow the fluid to bleed off, if machine moves now the problem is the master cylinder not allowing return, bent crimped tube/line. If brakes still do not release the calipers are stuck or material has built up behind the pads causings it not to release and the only thing you can do is remove the wheels unbolt the calipers and repair them. Have you checked you parking brake for release also. Keep wheel chocks close by and use them while repairing. Brakes need to be fixed.