I guess the reason why i am struggling to understand you is i dont have a complete understanding of kubotas power beyond setup. The oil goes from the pump , to the fel valve. Then it is sent on through to the power beyond circuit, unless you actuate the fel valve? Then it goes through the cylinder and back to the tank? Or is the tank line just for pressure relief?
Fluid from the pump goes into the Pump port. If the controls are in neutral, then the Open Center just passes the fluid on to the PB port and from there to the next control in the chain. If that control is also in neutral, then the fluid is passed on to the next in the chain, and so on. If all valves are in neutral, then the fluid is passed by the last valve straight back into the reservoir, and no (well, very little) pressure is developed in the system -- the pump is simply cycling fluid around the system.
With PB: If one of the control spools is activated, then fluid from the Pump port is diverted to the corresponding cylinder, and it moves. That is to say, the upstream spool valve cuts downstream valves out of the circuit -- it is not possible to use the 3PH when the FEL control is actuated.
As it moves, fluid from the opposite end of the cylinder is pushed out, and returns to the reservoir via the Tank port & hose. When the cylinder reaches the end of it's stroke (or capacity is otherwise exceeded) and the pressure rises above the relief valve setting, the relief valve opens and dumps excess fluid back to the reservoir via the Tank port & line, preventing damage like burst lines, cylinder(s) or pump casing.
The critical factor here is that the relief valve responds to the difference between inlet pressure and outlet pressure. So long as the outlet pressure is nearly 0, the relief valve limits upstream pressure to it's setting. More on that in a moment.
Without PB: If open center spool valves are chained in series without PB -- that is, with just the two hoses such that the Tank port from the upstream spool valve is connected to the downstream spool valve Pump port, then when one of the upstream spools is activated, fluid from the Pump port is diverted to the corresponding cylinder, etc. as before. However, the returning fluid and the relief valve discharge are passed through the Tank port to the downstream valve instead of directly to the reservoir. Now, so long as the downstream spool valve(s) are in neutral, things will work as expected since the fluid simply passes through open centres and back into the reservoir -- just as it would if it was passing through a dedicated hose direct to the tank from the upstream valve in a PB system.
The problem arises when both downstream and upstream valves are actuated simultaneously (possible only because the downstream system is supplied by the discharge of the upstream system). Consider the case of simultaneous operation of a FEL upstream and 3PH downstream without the use of PB and the third hose:
Let us say that the system is designed for a maximum pressure of 1800psi and therefore both the FEL and 3PH relief valves are set to 1800psi. The 3PH valve is operated, the Open Centre is closed, the 3PH rises to maximum and the relief valve operates. Pressure between the pump and the relief valve has now risen from 0psi to 1800psi. At the same time, the FEL cylinder hits it's maximum extension. The FEL relief valve sees a pressure difference of 1800psi inlet - 1800psi outlet = 0psi, so it does not open. It will remain closed until the pressure rises to 3600psi inlet - 1800psi outlet = 1800psi. This is called compounding the relief valves. Of course, there's a good chance that something will fail -- catastrophically -- before then!
How likely is the above? It is an example of the perfect storm. If at any time the 3PH control is returned to neutral, the Open Centre of the spool valve will immediately start dumping fluid to the reservoir and system pressure will again be limited to the 1800psi setpoint of the FEL relief valve. There are few if any situations were someone would deliberately operate both controls simultaneously -- especially if the valves are operated by the same hand. But it is a real possibility that one valve might be operated inadvertently and if that happens, the pressures will rise faster than humans can react. The PB port and 3rd hose are there to prevent the possibility of irreversible and expensive damage.
And in case it is not yet obvious, the last set of spool valves do not require a PB port or third hose because all fluid from that point on is going straight back to the reservoir -- it is not possible to compound the final relief valve.
Clear as mud? <lol>