Hydraulic coupler won’t connect: how to relieve the pressure trapped in the hose

When an implement hose will not go into the outlet on a tractor or skid steer, look first at the oil trapped behind the coupler, not at the coupler. A quick coupler has check valves that shut when the line is pulled apart and hold the oil behind them under pressure. Until that pressure is released, the tip will not seat.

Hydraulic coupler won’t connect: what is stopping it?

A coupler that will not connect is held shut from behind: the tip cannot push the check valve off its seat against the oil behind it. A 1985 US patent for a pressure-releasing tool puts it in two sentences:

“The check valves maintain the fluid under pressure in the valve assemblies and lines. This fluid pressure must be relieved before the lines can be connected to the couplers.”

US Patent 4,499,796, 19 February 1985

The pressure can sit on either side. With the control lever centred, John Deere’s manual for the 5310 S tractor says, the remote cylinder is “hydraulically locked in position”. An Ohio State University Extension training sheet adds that fluid can stay trapped “even when the engine and hydraulic pump are stopped”, and that a raised implement can hold pressurised fluid even when it is disconnected.

Heat does the rest. The same sheet: “Heat causes the fluid to expand, increasing the pressure.” It adds that the pressure of trapped fluid can be more than 2,000 psi. On a tractor forum, owners describe implements uncoupled cold and left in the sun that will not couple once warm.

How do you relieve hydraulic pressure on a tractor?

On the tractor side the pressure is released with the engine stopped, by working the control lever or putting it in float. The Ohio State routine: engine off, brake on, implement on the ground, control lever moved back and forth several times.

Float does the same job where the valve has it. In John Deere’s manual for the 110 tractor loader backhoe, moving the lever past the detent into float relieves the pressure on both quick couplers.

On an electrically switched outlet that may not be enough: on some electro-hydraulic controls, a forum reply notes, the ignition has to go back to run without starting. The 110’s third outlet is of this kind, and its manual has a routine of its own: engine off, key switch at RUN, outlet switched on, then each hose connected with the flow switch set to its side. Another tractor’s sequence is in its own manual. The 110 manual’s caution: “Do not attempt to connect hoses to machine SCV couplers until all hydraulic system pressure has been relieved.”

How to relieve hydraulic pressure on a skid steer?

A skid steer or compact track loader has its own pressure release, and it differs by model. On Bobcat’s T190 compact track loader, the front auxiliary couplers release the pressure themselves:

“Push the quick couplers tightly together and hold for five seconds; the pressure is automatically released as the couplers are installed.”

Bobcat, T190 Operation & Maintenance Manual, September 2006

Disconnecting is the same movement, followed by pulling the sleeve back. The rear auxiliary couplers have a pressure release button on the instrument panel. Before an attachment comes off, the manual wants it flat on the ground, the engine stopped and the auxiliary circuit relieved.

This is one model. A Kubota, a Cat or another Bobcat has its own routine, in its own manual.

What can be done on the implement hose, and what not?

No lever on the machine relieves the implement side: as one forum reply puts it, float does nothing for pressure built up in the attachment. What can be done is to remove the cause: rest the implement on the ground so that no cylinder carries a load and, if it has stood in the sun, let it cool. Forum owners wait, or run cold water over the hoses.

The field is cruder: a rag over the male tip and a push against the frame, a tap with a hammer, or a spanner on a hose fitting. The 1985 patent describes what striking costs. Striking the check valve on a hard, rough surface, it says, can damage the valve’s outer surface until it will “leak hydraulic fluid when in the closed position”. And the oil sprays: “The spray is not controlled so that it can contaminate the fixed object and the operator.”

The forum is divided: some owners tap the tip through a rag as routine, and one refuses because he expects the oil to come straight at him.

Apart from Bobcat’s self-relieving couplers, the manuals we read give no method for relieving the implement side. The conclusion is ours, not theirs: deal with the pressure when the implement comes off, not when it goes back on.

How do you leave the hoses without pressure when the implement comes off?

A hose uncoupled with the implement on the ground, the cylinders unloaded and the outlet relieved has nothing trapped in it. John Deere’s 5310 S manual gives two steps: retract the remote cylinder as far as possible, to protect the rod, then pull the hoses with “as much hydraulic pressure relieved as possible”. Forum habits add two more: uncouple with the lever in float, and couple the implement’s two hoses to each other.

Pressure relieved and it still won’t connect?

With no pressure behind it, a tip that will not go in is dirty, damaged or the wrong pattern. Bobcat’s manual has dirt removed from the faces of both couplers before connecting, and a coupler replaced if it shows corrosion, cracking, damage or excessive wear. Deere’s 5310 S manual says the outlets take a standard hose tip as recommended by ISO and SAE, and that older John Deere tips need an adapter.

Thread types at the hose end are covered in our article on fittings and threads. A hose in the wrong outlet is not a coupler fault: the Ohio State sheet wants lines taped or colour-coded so they are not crossed.

What order should you try things in?

1. Implement on the ground. Nothing raised, no cylinder carrying a load.
2. Machine side. Engine off; lever back and forth, or into float. Skid steer or electric outlet: the release in its manual.
3. Clean and push. Wipe both halves, push the tip in firmly.
4. Still not in? The pressure is on the implement. Let it cool. Do not hit the tip or crack a fitting.
5. Then the hardware. Damage, wear, wrong tip pattern, crossed hoses.
6. Next time. Uncouple on the ground with the outlet relieved; cap the ends.