One wheel cylinder expands on its own.

All new brake system on my 1965 Chevelle,
converted to a power dual master cylinder,
has a right rear wheel cylinder which expands
on its own, even with the drum off, car on
jacks at 40 mph. The push rod to the
mastercylinder has been backed off so no
constant pressure, no heat on the line as the
cylinder expands after 10 minutes, though the
line and reservoir fluid is cold. The brakes
have been bled numerous times with DOT 3.
The same line is in common with the left rear
cylinder which functions properly.