Just about every cyl if fully extended or retracted will activate the relief valve, if the cyl and valve are good, you may or may not hear the sound, depending on ambient noise around you. However, if a hyd gage is in the circuit, you will see the results of any pressure in the system. The pressure readings may not make sense at first, because you will see low pressure, but the lift and tilt are working. As pressure is developed because of load, the pressure gage will reflect that.
You can use the gage as a load measuring device by the pressure . At 3000 psi, you can lift 800 lbs, and if the load is about 400 lbs, it will only take about 1500 psi to raise it.
The cyl and the position of the cyl pins, are designed for a load factor, and for the 425, it is around 800 lbs. The relief valves are a safety valve for over pressure, and are usually set about 50 to 100 psi below the pump max pressure. Looking at the gage , you will see any pressure between 0 and the relief pressure. When you max out the cyl , the relief will go off and the pressure is reflected on the gage. That reading is a good reference for the ability of the hyd system to preform it's task. If the relief pressure should be lower than specs, it can be adjusted. Your pressure might be around 2900 psi.
Some PT owners say the relief never goes off, and the machine will go into the PT Pucker, which is a balance of the load fwd of the front wheels, and the rest of the machine. If the load is more than the rear, the PT rear end will stay in the air.
Can the 425 lift more, probably, if there is some more weight added to the rear, but one must give thought to the load on the wheel motors shafts and mounts, and more air pressure in the tires.
If in the Pucker mode and you want to come down quick, just hit the float. If you want to come back to level slowly, ease on the down lever. Sometimes turning the steering will cause the Pucker, because of weight shifting.