Interesting ingenuity n175h. Keep in mind though, that brass ball valve is not rated for hydraulic pressure and can burst. One thing you could do to prevent bucket drift down caused by faulty control valve would be install either a pilot operated check valve or pilot operated counterbalance valve at cylinder lines. These valve would lock oil in the side of cylinder that drifts, and would only open when you operate the lever to make bucket tilt down.
Do you have one or two bucket tilt cylinders? Could you post pics of them?