656 Loader Problem Update

Hello,
I posted sometime back about our 656 loader quit working this winter and needed work. I finally am getting around to working on it. I changed the filter and fluid in it (ouch $16 gallon for Hy-Tran). I bought 16 gallons because that"s what the book called for. I pulled the two drain plugs in the middle (TA housing and the front of the transmission). Then I pulled the plug out of the rear axle housing.) However, When I went to fill it back up, in only held 11 gallon instead of 16???? I moved the steering wheel back and forth as suggested by the manual. Steering works fine on the tractor, but the Hydraulic fluid level stayed on full at the dipstick.
As I said, the steering works good and the hitch seems OK although, I have not hooked it to a heavy
Implement. I put a pressure gauge to front & back remotes and was getting pretty much nothing. After you throttle it up a little, the best it gets is 350 PSI. I pulled the center link plate off the back and looked inside the rear end but really didn"t know what to look for. I listened for suction but tractor was in a lean-to and too noisy for my deaf ears to hear. All I saw was the oil stirring in the bottom of the rear axle housing and a stream pouring down from the top and I"m assuming this is normal
Is there any good way to check pressure relief valve? If there is sure don"t know what it is.
I"m pretty sure it"s the pump but I want to make sure before I spend that kind of money. I"m open to co

First off if it is a gear drive it will hold about 11 gallons and oil level is checked with engine shut off. A hydrostatic drive tractor will hold 15 and is checked while engine is running for at least a few minutes to fill hydrostatic case. It drains back when shut down and shows over full. Now as far as fluid coming down from the top inside, it should not. The return flow from pump or pumps is directed through that pipe and hose directly back into the main suction tube in bottom of rear. Either you have a broken hose, tube, blown O-rings or relief valve is stuck open for oil to come down from top when no valves are dead headed. If steering works good, main pump is ok.

That"s interesting about the gallons of oil it holds. My Case/IH Manual says Gear Drive Farmalls hold 16 gallons and the Hydrostat Drives hold 20 gallons. So, the manual must be wrong because with the engine off, it"s full at 11 gallons. I"m also glad to hear about the main pump. Now if I could just track down where the problem is. Thanks for the good info Pete. This helps a bunch.