[Kurt in NJ]

Those “big things” are usually in the end of the bore of single master cylinders.
Most of the dual master cylinders don’t have residual pressure valves — there was a change in the wheel cylinders— instead of just a spring a tapered expander was added to the spring to put pressure on the edges of the cup to keep them in contact with the walls of the cylinder taking over the function of the residual pressure valve.