Hello Lieuwe.
Nice work so far.
I like this dusty look of your ZiL.
I have also ZiL-131 (from ICM) on my workbench.
If I may, I have some observations (if you want an accurate model):
1. ZiL-131 had (still have) 3 air reservoires. The kit (Revell, ICM) provide only 2. The third is located under/behind the step ladder of the cabin on the right side (see ZiL-131 from Bronco Models CB35193);
2. The fuel tank holder (both sides) doesn't have a flat bottom. There are only two support beams at the both ends of the holder (but I think is too late for this modifications).
3. 5 of 6 tires are mounted opposite. The tip of "V" mark should face forward (at all type of tires and vehicles).
I hope the observations are useful to you.
Keep up the good work!