hydraulic question when using combo bucket

[yellowdogsvc]

I've got an F series with the Doosan motor. I've had Toolcats since 2005 and never noticed this issue before.
When using my 4n1 or combo bucket and when I open it, it bucket wants to drift closed. If I click the hydraulics on, the bucket will stay in the open position. When I release, it drifts closed. The boom arms do not drift at all and the dump bed doesn't drift down.
I tried the bucket on my s750 and it worked fine. I then tried the 750's combo bucket on the Toolcat and it did the same thing.

I recently changed hydraulic filters and everything else seems to work as it should.

Has anyone had an issue like this? In the past, symptoms like this would be a leak somewhere in the attachment but the cylinders are new and the other combo bucket is new. Could this be an issue in the auxiliary hydraulic coupler block that's on the boom?

Thanks for any ideas.

 

[SSdoxie]

If neither bucket clam drifts on the 750 and both do on the Toolcat, leakage is Toolcat Auxiliary valve.

 

[yellowdogsvc]

If neither bucket clam drifts on the 750 and both do on the Toolcat, leakage is Toolcat Auxiliary valve.

Dennis, thank you. Is that a big repair? Do you think it's a coincidence that it worked prior to changing both hydraulic filters (high flow machine)?
I wondered if I got air in the system or if it breathes from tank cap..