The hydraulic systems of the side console and pre side console tractors are quite a bit different. The pre side console tractor depends on a check valve deep in the transmission to keep the front oil cooler and reservoir filled. Many have worn out. In my I&T manual to check that valve is to destroy it. I found there was a JD service information about converting an elbow to a new check valve. I have created a web page about it.
When that check valve is bad, hydraulics (including steering and brakes) are lost with the clutch pedal pushed.
My '68 gas 4020 had that problem. I fixed it several years ago and it can sit for months, I can start it with the clutch pushed in and lift the loader without running the transmission pump at all.
Side console 4020 seem to have more problems from pressure leaks than a failed check valve.
The front pump is expensive to fix, and it seems like its most common failure is a cracked case that is not fixable. That shows up with a puddle of oil under the pump.
Changing the filter and cleaning the suction screen and making sure there's really oil in the system, not oil plus water are the first steps to be taken.