Slow, Creeping Clamp Cylinder with a Bypassed Filter

Start with what the readings already tell you:

• The cylinder is stalled and the pump outlet shows 2,100 psi against a 2,500 psi relief setting. Pressure is being lost somewhere between the pump and the dead-ended cylinder. There are only three places for it to go: past the relief, back through the pump internally, or past the cylinder piston seal.
• A popped bypass indicator and dark oil mean the fluid is contaminated and unfiltered flow has been circulating, which makes all three failure modes more likely - not less.

Discriminating tests, in order of information gained per unit of downtime:

1. Pump test, isolated. Install a flow meter and load valve at the pump outlet, bypassing the machine circuit. Measure flow at 500, 1,500, and 2,500 psi. A healthy pump holds its rated flow up to the relief setting. Flow falling off sharply with pressure points to internal pump wear. On a gear pump, also measure case drain flow.
2. Relief test. Tee a known-good gauge upstream of the relief and slowly raise the load valve. If cracking pressure is well below 2,500 psi, or if the relief passes flow at pressures below its setting, the relief is the loss path. A contaminated pilot orifice will do exactly this.
3. Cylinder test, isolated. Block both cylinder ports with rated caps and pressurize one side with a hand pump, or disconnect and cap the line and see whether the rod drifts under load. Movement with the ports blocked means the piston seal is bypassing. Also check the rod for scoring and the wiper for damage, since contamination in the fluid will have reached it.
4. Verify the gauge itself before believing 2,100 psi; a permanently mounted gauge in a hot, vibrating line is a common false reading.

Only after these isolate the loss path should parts be ordered.

Recommendation: do not run the clamp circuit as-is.

Why the answer is not "run it and see":

• Safety. A clamp that will not hold means the part can move or eject during the dwell. On a molding press that is a struck-by and pinch hazard, and the interlock logic may be relying on clamp pressure. That is a stop-the-job condition regardless of schedule.
• The failures are multiplicative, not additive. A filter in bypass plus 175 F oil plus a dirty suction strainer is a wear engine. Every hour of running sends abrasive into the pump, the relief, the valve spools, and the cylinder seal. A pump replacement today becomes a pump, valve, and cylinder rebuild if you wait.
• Heat degrades the fluid. At 175 F the oil oxidizes faster, viscosity drops, and internal leakage increases, so the machine degrades further as it runs.

Fix now, whoever the failed component turns out to be:

1. Replace the return filter element and confirm the bypass indicator resets.
2. Clean or replace the suction strainer and check the suction line for air leaks.
3. Find the heat source: cooler flow, cooler fouling, relief passing continuously, or a setpoint too high. Do not just open a window.
4. Take an oil sample for ISO cleanliness, water, and viscosity, and flush or change the fluid based on the result. Dark fluid in a sight glass is not a color question, it is a particle count question.
5. Fix the leak path, then re-run the creep test and record it as the new baseline.

Energy and stored pressure:

• Lock out the pump drive and any auxiliary pumps; do not rely on the selector switch.
• Bleed accumulators per the manufacturer's procedure and verify zero pressure with a gauge, not by cracking a fitting. An accumulator can hold pressure long after the pump stops.
• Relieve system pressure before breaking any connection, and expect trapped pressure in cylinders, lines, and pilot circuits.
• Block or mechanically support the platen or clamp before working under or around it. A cylinder can drift even with the pump locked out.

Fluid injection and burns:

• Never check for a leak with your hand. A pinhole at 2,100 psi will inject fluid through skin, which is a surgical emergency. Use cardboard or a mirror.
• Wear eye protection and gloves, and know where the nearest eyewash is. Hot oil at 175 F will burn.
• Only use rated gauges, hoses, and fittings. Cap every open port with a rated cap, not a rag or a plug.

Contamination and housekeeping:

• Work clean. Clean the area around any opening before you open it; a reservoir opened in a dirty shop reintroduces the problem you just fixed.
• Contain and dispose of drained fluid per plant rules, and clean up spills immediately - oil on a walkway is a slip hazard.

Coordination:

• Barricade the test area. Someone will be reading a gauge near a moving cylinder, and the clamp can move during a test.
• Use a second person as a spotter during deadhead and load-valve tests, and keep the test short - deadheading a pump generates heat fast.