Farmall 450 Hyd pump and system

Our 450 if it sets for a couple days will lose all Hyd function. When the small screw is loosen on top of pump air bubbles are in the system. After about 4 teaspoons of oil escapes
the system builds up normal 1200 lbs of pressure.
Do we have a leaky imput shaft seal,etc. Level of oil in reservoir is full.Thanks for any help.

You likely have a Oring or pinhole air leak in the suction line to the pump. Putting pressure in the reservoir (30 PSI max) and using some dishsoap in a utility bottle sprayer, at one teaspoon per 12 OZ, will show up a leak, if it does not show up from oil. Some bad Orings leak into the line, but not out. Cracked end fittings are also possible. Jim

Is the oil level inthe engine constant ? If it keeps "making it's own oil" and the dipstick is always over full, the front pump seal may be leaking hydraulic oil into the crankcase, if the back one is leaking the distrbutor will show signs of oil intrusion. Do the cylinders leak down when sitting a while? Have you noticed any oil leaks around the valves, couplers or lines?
I beleive it getting air in the system as it sits from a (or many) minor leaks. After warming up the tractor I would cycle all cylinders all the way up & all the way down several times to work as much air out as possible, collapse all cylinders, shut the tractor off & recheck the hy-trans fluid level in the reservoir & fill to the full mark. Wait a few hours & start her up, and go from there. Usually (not always) this will "burp" the air out of the system and the pump should start to pick it up at start up. The longer it sits, the more air will enter the system, this may become a ritual until all the minor leaks are spotted & fixed. They have a lot of different sized orings throughout the sytem, let alone replacing the lines & tightening all the fittings. I would first try to re-seal the pump, refresh the pick up tube & pressure line orings at the bottom. Place a small piece of cardboard between the pump lines & the frame rail and keep track of how much hydraulic fluid is leaking, and how long it takes to leak a tablespoon or so.
If the lines are just starting to leak, remove them & have them pressure tested & tig welded or brazed by a hydraulics shop. I had to make a "bracket" for mine to hold them together about where they meet the sheet metal on that side to keep the vibration under control.
An old timer told me about that little modification, he worked for IH back in the 50's & 60's at a delaership as a mechanic.
Sometimes it pays to take your tractor to a not-so-local show & start up a conversation with someone just looking at your tractor...and try to do a bit more listening than talking.

There is also a cone shaped screen inside the reservoir that comes out with four bolts right next to the dipstick. Drain the reservoir first & buy or make a new gasket, clean the screen in parts solvent & blow dry (very carefully) & re-install after cleaning as much goop out of the bottom of the reservoir as possible. Re-install & fill with hytans. Mine had quite a bit of water in the bottom, used parts washer fluid & several rags to get it cleaned out.