[Rust_never_sleeps]

On old earth moving equipment, they'd sometimes use an oil can clamped to a hose to bleed hydraulic systems from the lowest point back to the reservoir.
You might try this with a big syringe from the farthest wheel cylinder. Looking for flow, and maybe trapped air. This way is messy, so rig up a drip pan and wash off any spilled fluid in the engine compartment

Otherwise, I agree with the above and you'll have to get clever with swapping parts or plumbing to isolate the problem