Downhill hydraulic problem

My '86 7245 loose hydraulic power when it is pointing downwards. I understand that this is a common Zetor weakness, but how do I deal with it? Otherwise the hydraulic works fine(maybe a little weak), exept that it leaks quite a lot. I can see a leakage from underneat where the hoses for the loader is connected, but I suspect a bigger leak somewhere when load is applied to either the loader or the three-point.

Top up the gearbox oil

Or fit a front hydraulic pump

Ok, thanks,
I guess I will try the first tip first, even if I top up the oil regularly. I know from the manual how much oil the hyrdraulic system "needs", but, there are three marks on the oil-dipstick. I guess the lower is minimum, but what about the second and third? Which is the top mark? Is it with or without a loader? And then the classic question, do I mesure the oil when the dipstick is screwed down or when loose?
And why mount a second pump? Is it because the original one is weak/bad or broken? Maybe it is time to replace it. It has only 2000h. but the tractor has been used for snowploughing with a snow-blower for many years here in Norway and its heavy work even if its the pto which does the hard work.

Fill to the top mark . Filler not screwed down.
Pump cavitates when giong down a very steep hill because the oil pickup is at the back of the tractor and all the oil runs to the front of the gearbox.
When a front pump is fitted the old pump is not used and the oil for the front pump is taken from the bottom of the 4wd box which is in the centre of the transmission.
It's not a good idea to overfill the gearbox by very much as it will eventually get oil on the brakes and clutch if you do.