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catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 2244
Experience:  Field service technician for CAT 19 years.
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I have a cat 420d both swing cylinders were leaking hydro. I

Customer Question

I have a cat 420d both swing cylinders were leaking hydro. I rebuilt the cyls like before, under gravity, the boom will swing side to side, without using the controls. if you lean the machine one way or another, the boom swings. is there a holding valve
and a way to test it? thanks kevin
Submitted: 1 year ago.
Category: Construction and Road Equipment
Expert:  catmastertech replied 1 year ago.

Hello, I am sorry no one could help you earlier. I will be glad to help if you still need it.

Are you saying the problem is the same after resealing the swing cylinders?

Were the cushion pins or springs damaged in the bolt?

Are you certain you did not cross any lines?

Yes, the circuit has relief valve but, it should not fail in both directions. A load check valve is located between the hoses from the valve on some circuits. They have been found loose before.

I am not sure this model has them. If you can post the full machine serial number, I will be glad to research this deeper to help you.

Customer: replied 1 year ago.
everything looked fine. I know the lines are correct. we have 2 exact machines and I double checked.
cat0420dtfdp22697 is the vin.
thanks
Expert:  catmastertech replied 1 year ago.

Thanks for the reply. I have a couple of items you should check.

First, your machine should have a Crossover relief valve just to the right of the backhoe control valve.

Remove the relief valves (one top and one bottom) and inspect them for trash sticking in the end or ports. Clean as needed.

Next check the line reliefs on the backhoe swing valve. (Follow the lines from the swing cylinders to crossover relief and then to the swing control valve.)

Remove these reliefs and inspect them for trash.

Next, remove the bottom spool cap so you can access the primary resolver check valve. Inspect it for blown seals or debris sticking in it.

Finally, remove the flow control spool from the swing valve. It should slide out easily. If stuck or the spring below it is damaged, you may need to replace the spool, spring and or the swing valve.

I will attach a picture to help you identify the swing valve parts. The crossover relief valve will look similar to the line reliefs of the swing valve.

Expert:  catmastertech replied 1 year ago.

Please let me know what you find and if you need more help.

Customer: replied 1 year ago.
are the relief valves on the rear control block, where the pressure lines to the swing cylinders come from?
are all these spools and relief valves on the same block?
the pressure lines to the swing cyl's have a tee and small hoses go to the outsides of the machine.
what are these?
thanks
Expert:  catmastertech replied 1 year ago.

The crossover relief valve is to the right of the backhoe control valve bank. It is separate from the valve bank.

The swing control valve in the illustration is in the middle of the valve bank. The large lines that come out the back are the supply lines to the cylinders.

The small lines that come out the top and bottom are the pilot hydraulic control lines. These small lines connect to the hand control valves. This system replaces the mechanical linkage that use to control the valve bank.

The lines from the tee fittings are to the crossover relief valve, some were installed in different locations. Yours could be on the left side, it depends on the machine options on the exact location. This was also called a cushion relief valve.

The bottom cap for the main spool will have the small pilot line going to it. The flow control spool is on the top side and will only have a plug over it.

Be sure to keep everything clean to prevent dirt entering the system.

If you need more help, feel free to ask. I will be glad to help all I can.

Customer: replied 1 year ago.
hi there.
I have checked and cleaned all of those valves and there is no change.
anything else I can try?
Expert:  catmastertech replied 1 year ago.

I am guessing you did not have this issue before you worked on the cylinders?

If so, go back and recheck the piston retaining nut and cushion valve.

You could have a loose nut, damaged cushion pin or piston seal.

If you have gauges, test pressure to the cylinder by installing the gauge in the end of the hose to the cylinder. (Switch to other hose and test again.) Apply pressure and see if it hold pressure. If it does not hold pressure, the issue is in the valve. If it does hold pressure, the issue is in the cylinder.

Customer: replied 1 year ago.
I had this problem before I rebuilt the cylinders.
so I should check the pressure coming to the cylinder and see if it holds, then check the other.
Expert:  catmastertech replied 1 year ago.

Yes, that is what I would do to determine which direction to focus my efforts on.

Use the hoses from the valve by removing the other end from the swing cylinder. Install a plug in one hose and install the gauge in the other.

Swap the gauge and plug to test the opposite direction too.

Also remove the small lines to the crossover relief valve, plug the lines and cap the fittings to isolate the valve.

I am not sure if you have tried removing the crossover relief valve from the circuit to see if that was the problem.

If not, remove the small lines from the fittings at the swing valve. Plug the lines and cap off the fittings then test the swing operation.

This will determine if the crossover relief is the problem.

Please let me know what you find.

Customer: replied 1 year ago.
I have not removed the crossover valve from the circuit yet.
I will try that and see if the swing cyls. hold.
Expert:  catmastertech replied 1 year ago.

Ok, thanks for the update. Please let me know what you find.

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