Komatsu wa320-1 losing brakes over night

Working on a komatsu wa320-1 and every morning we have to refill the brake reservoir and bleed the brakes the mastereo cylinder was rebuilt by the previous mechanic when this problem first came up so my suspension is its leaking past the Pistons in the brakes. I tried blocking off the brakes one axel at a time but both times came up with the same result empty reservoir and no puddle on the floor. Is it possible I need to do the brakes in both axels or am I missing something

Komatsu wa320-1 losing brakes over night