The hydraulics in John Deere 4020 are bled when the bucket is raised and slowly lowered when the controls are released. The loader will only pick up the front of the tractor by rolling the bucket, but has the ability to do so with its arms.
Also when you open the round baler door it closes slowly.
I have checked that the fluid level is good and there are no signs of water.
So the pump is getting weak or I am missing something like a clogged screen somewhere.
This tractor is new to me. The loader is connected to the rear couplers.
There are several things that can cause your tractor’s closed-center hydraulics to be sluggish. Yes, the transmission oil pan screen may be clogged. It can be inspected / cleaned by removing a plug on the left side behind the hydraulic filter and in front of the rear axle housing. One needs to drain the hydraulic oil or insert the nozzle of a vacuum cleaner into the hydraulic fill hole to create a vacuum so that the oil does not leak out when the plug is removed.
If you don’t know how long it has been, I suggest you change the hydraulic filter (part # AR75603 $ 10.64). I also suggest that you check the reserve hydraulic pressure by plugging a 5000 # gauge into the rear brake and moving the control lever to activate the gauge. The factory pressure was 2250 psi but can be adjusted to 2350 psi without damaging the system.
A large internal leak can cause hydraulic functions to operate slowly by stealing from the pump outlet. If the transmission is in park or the brakes are being applied while trying to raise the front tires with the FEL, this will cause the loader to have to work harder because the rear tires need to roll forward a bit while the front tires are being raised. .