#1 dipstick is in back under cab
#2 Screen is to the rear of 2 Hyd. filters on side of case. Plug will be 1/2 inch square drive or a hex. Remove plug pull screen out. 
#3 Change filters and be sure to get o-rings for the housing. 2 filters, 1 is main one is power shift.  
#4 make sure coupling is not spinning on front pump shaft,
     Check for flow from line that feeds front pump, It runs along left side of frame rail, disconnect before the valve body under battery box, should put out a steady flow of oil during a very brief startup. You should get like 3 to 4 gallons in 20 seconds. Check same line for flow after valve body. There are ports on valve body that can be used for checking pressures, low pressures can indicate internal leakage but first you need to know the front pump is being supplied with oil. 

On the back of the tractor under the clevis for the third link there is a plug, if you remove the plug there is a hex driveshaft that turns the rear pump, pull this drive shaft out and check for a worn (rounded) spot, Never had it happen but heard of it. Rear pump supplies oil to front hi pressure pump.

(EDIT) Disregard  this suggestion,  TJG says below it is not applicable to power shift transmissions. 

OK, I did not know that.

I'll try. Power-Shift tractors have two hydraulic oil filters. The front one filters oil from the transmission pump that has sucked oil from an inlet screen in the bottom of the transmission case. That screen and suction passage is behind the second filter housing at the front of the left final drive housing right at the bottom of the case. There is a flat faced plug with a 1/2 inch square hole that threads out of the case. Smack it with a hammer and it will loosen easy. Be prepared for about 10 gallons of oil if you have not already drained the system. The screen is behind the plug and will require a pretty good pull to remove if it has much debris on it. The rear filter is in the function return circuit and filters oil from the selective control (remotes) valves and three point hitch. That oil is then returned in to the supply side of the system to the front pump. Your transmission pump ( low pressure) is driven live from the transmission input shaft and supplies oil to the Power-Shift clutch and brake packs and engages the PTO clutch before any oil is sent to the front (high pressure) pump. The Power-Shift and PTO will operate with the front pump completely removed from the tractor. Pressure gauges for testing the systems are your friend here. Hope this helps.

Only gear drive tractors have the hex shaft that removes from rear. Power-Shift transmission requires split at clutch housing for pump repairs.