Hydraulic System How it works??

I have an Case 430 with a front loader and 3pt hitch. The hydraulics are strong when I first begin to use them. After a few minutes they slow down and eventually stop all together. When I shut the tractor off and immediately restart it they will work again for a short time.

I have already checked the filter and it is new & clean. Also the pick up tube has no screen on it so I know it's not plugged. I'm having trouble diagnosing it because I'm not sure I understand how the flow on this system works. Can anyone give me a short description on the flow?

Is there a lift pump and a High pressure hydraulic pump? Does it continuously make high pressure or only when I am trying to lift? Does it return all the fluid back to the res or is it a closed loop? Where is the fluid being filtered? Before of after the pump?
Thanks for any help!

two things, first these models all had a open center hyd system, that means oil is flowing from the pump at all times then depending on whether you stroke a valve or not this determines where the oil flows, if you have a "CK" unit with a factory CASE loader then you have a front pump with a separate system from the 3pt hyd system on the tractor, if the system is starving from oil, (plugged vent/restricted oil supply/collapsed hose foreign object in tank) it can act like you describe, we need a bit more info on how it acts ect to pin point the issue, if you have a regular AG version it will have the same type hyd system it will just have one instead of two like a "CK" with loader and 3pt

Thank you so much for sharing your knowledge! I am almost certain that I have the AG. I have had several people tell me that all symptoms point to a restriction to the pump. But I don't understand how that would be possible since there is NO screen on the pickup tube and the fluid level is full.
When you suggest that it might be the vent, are you talking about the reservoir vent? I don't understand how that would cause this problem, but again that may be because of my lack of understanding the plumbing.

I'll describe the symptoms more:
Upon initial cold startup the Hydraulics are very very strong and fast. That old tractor can lift way more than it should! Slowly the speed and ability to lift fade away, typically about 5-10 minutes after use. They will eventually fade to the point that they won't even lift the bucket. At this point if you turn off the engine and then immediately start it back up it will work again but won't last quite as long probably only a minute or two. That process can go on as many times as we do it. If the tractor is parked for a long time it will give us a good 5-10 minutes of solid performance again.

I agree that a restriction sounds like a probable cause-- but I cannot think of a place where it could be restriced- or why it would operate well for the first few minutes.

Thanks

Who told you there is no screen on the pick-up tube? Every pick-up tube I have seen has a screen & I am sure that is your problem, a plugged screen.

I looked at the parts diagram and saw No screen, Just a tube. Plus someone told me they are screen free. What is the fastest way to look or get at it?