Kobelco SK70SR-2 Lost Boom Lift and Arm Extend After Low Fluid

Most likely culprit here is loss of pilot pressure, not the main pump. Boom and arm on this machine are pilot-operated (proportional joysticks feeding pilot pressure to the main control valve spools), so if the pilot circuit lost prime or is starved for oil, those functions go dead even though the main pump itself is fine. The fact that they came down and retracted but won't lift/extend fits a one-way restriction or a pilot supply issue more than a main pump failure.

Ranked causes and first checks, per the discussion on your own HeavyEquipmentForums thread:

1. Air still trapped in the pilot circuit despite clear oil at the fitting. As one commenter noted there, "Low oil can cause all sorts of odd hydraulic problems if it pulled air into the system," and "just because the oil looks clean doesn't necessarily mean all the air is gone." Loosening a line at idle only bleeds what's right at that fitting; it doesn't purge air trapped further upstream in the pilot pump or accumulator-fed circuit.

2. Confirm whether pilot pressure exists at all. As the same forum reply put it: "The other thing I'd want to know is whether you've still got pilot pressure. If the pilot circuit isn't building pressure, the boom and arm won't respond even if the main pump is fine." Since you don't have gauges yet, the cheap proxy test is this: "does it still swing and travel normally? If those work, I'd probably spend more time looking at the pilot side or the control valve before condemning the pump." If swing and travel are normal, the main pump and main valve are almost certainly fine, and this narrows it to the pilot pump, pilot filter, or the boom/arm pilot lines/solenoids specifically.

3. Control valve spool stuck or a check/holding valve not resetting on the boom and arm sections. Low-oil events can let a spool hang up or a load-check valve seat wrong, blocking flow to just those two functions while others work.

4. Main pump swash/regulator not responding — lowest probability given swing/travel would likely also suffer, but not ruled out without pressure gauges.

Concrete next steps in order of cost: get a basic pressure gauge kit (cheap, ~$50-100) and check pilot pressure at the pilot manifold with the engine at idle and at working RPM; if it's low or zero, focus on the pilot pump and pilot filter. If pilot pressure is normal, the fault is downstream at the control valve spools for boom/arm, which likely needs the valve pulled and inspected. Also, since you haven't run it yet: pull up the diagnostic/monitor menu on the display and check for any stored codes, some SR-2 monitors will flag a pump regulator or sensor fault directly. Beyond that, a mobile tech with pressure gauges and the KDT-equivalent Kobelco diagnostic tool is really the next real step; without gauges you're guessing at pilot vs. main pressure.

Sources:

• 2005 Kobelco SK70SR-2 Need advice from a Kobelco expert. Lost boom lift and arm extend. | Heavy Equipment Forums