My Kubota BX23S is a 2019 with 135 hours. He had ordered it with what Kubota calls a third-function valve and mechanical thumb options. I have converted the thumb to hydraulic and use that valve to feed the thumb cylinder.
I started reading the forum posts regarding PSI and ordered the BXpanded gauge kit. I tested it today and am getting a reading of 2700 PSI at idle connected to the 3rd function valve outlet, it was much easier to connect there than the recommended outlet for the loader.
By converting a mechanical thumb to a hydraulic one you may have created a situation where a hydraulic cylinder may be pushing against a smaller hydraulic cylinder and doing so amplifies pressure in a circuit.
This is similar to the situation where owners destroy bucket cylinders by back dragging, especially with forks installed Pressure relief valve cannot control or protect a circuit once the circuit control lever is in the neutral position Imagine picking up a bucket of gravel with a FEL.
You can barely lift the bucket off the ground because the relief valve is limiting the pressure in the cylinders. Now take a shovel and, with the bucket slightly raised off the ground, add more material to the bucket. The pressure in the cylinders and hoses has to increase to support the added weight.
The relief valve cannot do anything. In the extreme, the cylinders or hoses will fail from excess pressure. Find another place on your machine to measure the pressure before beginning to make adjustments.