Pressure thrust in case of pressure balance bellow to be taken care of by bellow itself.
When previously I was trying to model bellow in Caesar with three convolution piece & two tie rod set as it build.
But Caesar do not give good result. Bellow in model do not take care of pressure thrust as it do for tied expansion bellow.

Then I was told to put effective ID of bellow zero to get zero pressure thrust & feed all stiffness values as supplied by Vendor.