So I have installed a new Tilton brand clutch hydraulic mastercylinder in my Nomad restomod. Custom fabricated mount position on the firewall, pushrod and pedal attachment under the dash.

Its plumbed using a stainless line down into the T56 transmission, with a stock hydraulic throwout bearing.

The clutch works well for the usual start/stop and upshift/downshift requirements of driving.

But when I push the pedal in, and hold it for extended periods (like 1+ minute), the throwout bearing seems to be slowly releasing...such that the clutch is no longer engaged. Almost feels like maybe the fluid is not "holding" the TO bearing in position after holding the pedal down for 1+ minute continuously.

I see no evidence of any fluid leaking out of all the connections in the system, and the small fill tank is full of fluid.

What could be allowing the fluid pressure to back off slowly?

When it does this and I release the pedal...the pedal will not return to it's normal position. It slowly backfills the pressure and resumes correct operation within about 5 to 10 seconds of releasing it.

Ideas on what's going in here? I thought I bled this thing of all the air when I initially filled it for the first time (a year ago), but maybe there's still air trapped in the system?