Yeah, I'm the one that documented the pump change in this forum and I measured the performance by timing the lift speed over 4 complete cycles and calculating the cylinder volume. The original pump was putting out 3 USGPM at full throttle, the "6 usgpm" pump was only about 20 or 25% faster. Better, yes, but not 6USGPM.
The diameter of the hoses does not affect the pressure (PSI or KPa) at all. It affects the flow rate (GPM or LPM). Like I tell the new recruits at work: small fire, small hose. Big fire, big hose.
Pressure is limited by the construction type - single reinforcing wire hoses can handle the pressures of a small tractor, 2 wire hoses more pressure, 4 wire hoses lots and lots of pressure. Look at the pressure rating printed on the hose and use that for guidance. Remember that higher pressure hoses are stiffer hoses, so more is not always better!
I used 3/8" (~10mm) 2-wire hoses for the pressure lines (Pump, PB and cylinders). They are rated 4,000psi and probably overkill. But they were available in complete assemblies at my local farm supply store and cheaper than buying separate 1-wire hose and fittings from the local hydraulic repair shop. I used 1/2" (~12mm) 1-wire for the tank line. Some experts recommend 1 size larger hose for the tank line, others say it does not make much difference and is not worth the extra expense. Smaller diameter hose is certainly easier to handle and accepts tighter bends, but then, the tank hose(s) do not have to be high pressure hoses, and low pressure hose takes tighter bends.
The relief valve in your new assembly should be adjusted to suit your tractor's designed maximum. Kubota designed the system to safely handle about 1700 psi. The pressure affects more than just the valves, hoses and cylinders. Higher pressures cause more force on things like the pump drive mechanism. The lifting capacity will increase, meaning you might accidentally lift more weight than the front axle can support.
I can't find a number for the lifting capacity of the tractor itself, but the lifting capacity of the B219 loader Kubota made for this tractor was about 500 pounds (~225kg).
Using a calculator like
this online tool, you can calculate the raw lifting force of your FEL cylinders.
Let us say that your cylinders are 1.5", with 1" diameter rods. And let us say that your cylinders attach half-way between the bucket and the support posts at a 10° angle from horizontal. The fulcrum of the lever supports twice the force of the load at the end of the lever, but you have two cylinders, so force = force x 2 / 2. At 1710 PSI, your system could lift 525 lbs (238kg), including the weight of the bucket, cylinders and arms. But at 2536 PSI, that jumps to 779 lbs (354 kg)!
Of course, you need to take your own measurements. Changing one small thing can make a big difference. For example, if the cylinders are 2" instead of 1.5", the lifting force almost doubles. If the cylinder angle is 20° the lifting force more than doubles.
Even the distance the FEL sticks out in front of the tractor affects how much load is put on the front axle. The B219 loader is 514mm from the center of the front axle to the center of the pin at the end of the arms. If your loader is bigger and sticks out further, it will put more strain on the front axle when lifting.
For all these reasons, I strongly recommend you adjust the new relief valve to suit your tractor and FEL. Don't just leave it at the default setting.
