vacuum booster apart

this image shows the vacuum booster [1973] separated into it's front and back half. my method-clamp the alum plate that the master bolts to in your vice, take two large prybars or screwdrivers on the backside [part that bolts to the firewall] and-after applying some penetrating lube, carefully turn them counter-clockwise about 1/16 turn. mark the alignment position-prior to separating them!