Case 480d brakes locking down

I have to go look at my uncles case 480d backhoe. He says brakes are locking down. I tried to look at it eailer but battery dead not cable to jump in my truck I was in. I am a cat guy with sis access but never touched a case. I was thinking master cylinder but that's a blind guess don't know. It's serial is 5470963 thanks for any direction