Hidden: I think I used a Prince valve, which was around $150. I selected this one because it has a work port relief and also a power beyond port. Both of those are very important. I also selected it because it's a segmented valve and gives me the opportunity to add a second spool sometime in the future for something else. It's all detailed in the thread I referenced. Please read that since it has a tremendous amount of info about the design. I had to learn a great deal while doing this and all that is covered there.
The valve is inserted into the tractor power beyond line that runs the backhoe. So the tractor feeds the new valve, and the power beyond port in the new valve feeds the backhoe. So when you are running the thumb, it will interrupt the operation of the backhoe. I have found that when I'm lowering the boom, that this is a good time to extend the thumb if needed. I think the pressure created by the weight of the boom make it possible to make both motions at the same time. The work port relief is important because it creates a relief point for when the bucket pushes against the thumb. I was particularly concerned that when the thumb and bucket push against each other, that in some positions there could be mechanical advantage that could lead to excessive hydraulic pressure, and something could blow.
In my thread, you'll see the details of how the spool valve is mounted and how it's foot operated. I was going to add some kind of pedal, but the way it's set up now works perfectly and doesn't need anything more. I can lift to retract, and push to extend, which is quite intuitive.
