Hannu, I have exactly zero experience with the hydraulic front assist systems, but I have a service manual laying here and I think I may have found some possible causes of your problem. The heading is "Hub seal leaks or relief plug in oil manifold blows (pressure build-up in planetary brake housing)". Possible causes are listed as follows:
1) Relief valve packing defective
2) High leakage at wheel motors
3) Pivot pin drain tube seals leaking
4) Bleed lines restricted
5) Overspeeding of drive motors
6) High leakage at valve plates
7) Brake piston bleed valve leakingThose are what is listed, and I would have to agree that the problem appears to be too much pressure building up in the housings, which is going to be caused by more oil bypassing through the motors than the bleed-off lines can handle, or a restriction of the bleed circuit allowing excessive pressure to build up. Some leakage past the motor parts is normal and is required to provide lubrication. Some things that would be easiest to check would be that the bleed circuit is not restricted (no kinked hoses, plugged lines, etc) and then check motor leakage. I would lay out all the procedure, but it gets pretty involved, so your best bet will be to get a service manual and just start following the procedure. Do not skip any steps, as I have seen the causes of the problem skipped over when a person assumes that one particular step couldn't be the problem. Good luck with the 4230, and email me if there's anything I can help you with.
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