The pressure is maintained in the liquid cylinder through its normal heat leak of the cylinder.
The pressure will rise in the closed cylinder as its liquid contents boil off.
It is normal for the pressure to reach the relief valve setting of 22 psi (1.5 BAR) and vent slowly into the atmosphere. The transportation of the cryogenic products in these liquid cylinders is not regulated by the DOT/TC since the pressure is normally below 25 psi (1.7 bar).