Ive owned the big trucks for decades. Hyd brakes are great if driven regularly. However let them set a few months and the rubber componets dry out. the brake fluid absorbs moisture and then evaporates pitting wheel cyls and master cyls ect. Ive got several c 60,s that don't have any brakes. It would cost more than the trucks are worth to fix them. A new brake booster is over $300! Ive got a C65 that has full air brakes. and they always work.