ISX 475 oil leak

[s53s]

Hi, I am in South Africa, I recently purchased a 9800i international truck 2002 ISX 475
The truck is leaking on the tappet cover when it goes for trips 300km with loads 30 tonnes plus , whenever we fix the leak and test drive it without a load there's no leaking.
The driver says to me the leak gets worse when he puts Jake brakes on. The last leak we fixed caused the oil to come out through the breather instead,
A mechanic thinks its the rings the previous owner says he did the rings and sleeves 2 years ago and it can't be. Another opinion says it might have a crack on a piston.when the leak occurs its about 4litres of oil.

Please assist