During a BOOM LOWER operation, the oil delivery from only right pump (23) is supplied to boom
cylinders (1) through boom I control valve (9). Boom I control valve (7) operates alone. Boom II control
valve (18) has a regeneration circuit that functions during the BOOM LOWER operation. This function
supplies return oil from the head end of boom cylinders (1) to the rod end of the boom cylinders.
When the joystick for the boom is moved to the BOOM LOWER position, pilot oil from pilot control
valve (20) flows through pilot line (22). The pilot oil flow then divides into three flow paths. Part of the
pilot oil flows through port (5) to boom I control valve (9). Part of the pilot oil flows through port (21) to
boom II control valve (18). The remainder of the pilot oil flows through port (7) through boom drift
reduction valve (12) and the oil acts on spool (17) .
The pilot oil flow from port (5) shifts stem (27) in boom I control valve (9) against the force of spring
(28) and spring (30). The oil delivery from the left pump in parallel feeder passage (16) flows through
load check valve (15), passage (38) and port (6). The oil delivery from the left pump then flows through
line (2) to the rod end of boom cylinders (1)
The return oil from the head end of boom cylinders (1) flows through line (3) into boom drift reduction
valve (12). Since valve (17) is shifted by the pilot pressure from pilot line (8), passage (26) is open to
drain port (8). The oil pressure that is acting on valve (12) becomes lower than the pressure in line (3).
Valve (12) is shifted to the left and the valve allows the oil in line (3) to flow through line (11). Then, the
oil flows through passage (32) and return passage (14) to the hydraulic tank. Since the return oil is
restricted at passage (32), the movement of the boom cylinders is slowed and the boom can lower at an
appropriate speed in accordance to the flow rate of the right pump oil.
Pilot oil from pilot line (22) flows through port (20) of boom II control valve and the pilot oil forces stem
(34) against spring (29) and spring (31). Now, part of the return oil from the head end of the boom
cylinders enters boom II control valve (18) and the oil flows through passage (35) to check valve (19).
The check valve opens and the oil flows through passage (39) through port (6) to the rod end of the boom
cylinders.
When the joystick for the boom is partially moved to the BOOM LOWER position, stem (27) and stem
(34) are shifted upward slightly. Passage (37) and passage (40) are partially opened and passage (32) is
closed. Return oil in center bypass passage (10) flows through partially opened passage (37) to center
bypass passage (13). When passage (37) is opened enough, an optimum amount of oil flows through
center bypass passage (13). Now, negative flow control pressure destrokes the pump for proper cylinder
operation.
Nige posted all this, I guess none of us read that far down.
Let's focus on servo vs pump in regards to right hand pump or r/h pump. It states that servo splits into 3 places. Plus side oil transfers to negative side through boom drift reduction, if that passage hangs open , it's a path to tank.
I still believe the problem lies in the boom down circuit . If the path is open to tank, there will be no/low pressure. I believe that bringing the other function helps close the passageway. When your pump is at full stroke and you suddenly close the passage, it shocks the pump, that could also be why you can hear "shoe lift" insufficient back pressure on pressure side of pump
Whatever you do, please avoid hitting additional functions while boom down. We dont want to fail another pump.