Rene, have you tried just unbolting slightly the two bolts holding the MCto the firewall to see if your push rod is making too much contact with the MC? May be a simple troubleshoot approach, but I know on a 67 vert I had, when swapped the MC (supposedly same part number), the MC piston surface was closer to the rod.
Was hard to tell but the push rod was engaging the MC enough. I removd, adjusted the pushrod length turning it, and problem resolved.
Gui
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