valve slippage leaking seals

[Flycop2000]

Hi,
thanks for all your sugestions, I've checked for leaks and there is none that I can find.
I should have mentioned earlier that my excavator is a Metalhobi hydromek 490. I've not had it very long and can't remember if it always did this from new as I had a spell where I did not use it much due to other commitments.
If slow leak down is normal, I can relax and stop looking to fix a non existant problem.