FIXEDOk, took the return line from the filter to the pump off, plugged the port on the pump. Hooked up a bottle to catch the oil from the hose to the filter, started it and checked the gauge, no pressure. Filled the bottle in about 5 seconds so the restriction isn't in the hose. I then checked the elbow to the pump and its also clear.....
I'm going to pull the pump and motor on the weekend if the weather holds, i need to get this problem sorted out, its driving me crazy! I had a look at the pump i have for my 763 thats currently in pieces and there is a hole from the input shaft to the inlet of the pump. The restriction must be here somewhere or someone replaced the shaft bearings and left the seals on them.... I have no idea what the previous owner could have done.
Anyway, time will tell.
The problem was the previous owner changed the gear pump, it was a different brand that didn't have an oil feed hole from the hydrostatic pump through into the inlet of the gear pump. Without the port the pressure built up till the cold weather port opened up at 400 PSI. If that wasn't there the oil filter or oil cooler would have blown. It could have even blown the seals on the hydro pump!
I'm sure people will think I'm MAD but i got the engine and pump out, removed the hydraulic pump and took the plate off. I then drilled a 1/2" hole through and into the inlet side of the pump. This made the pump essentially the same as the one it should have had fitted.
Put it all together and its PERFECT, quiet as a mouse! the valve opens up when you lower the arms quickly which is exactly how it should work.
FYI - Removing the pump and motor as one is MUCH easier than it may seem. I removed the engine/pumps on my own this morning, repaired the pump and with the help of my dad and a mad mate that popped over, we had it back in, plumbed and wired up within an hour! Filled with oil and it came back to life. Its messy as you have to crack hydraulic lines but its not at all difficult, just build a lifting jig.