Field-adjustable, seal-less frangible downhole pressure control and isolation device and subassembly for conduit in wellbore

A pressure control apparatus, comprising:

• a tubular body with connection means enabling the body to be connected in-line with a wellbore tubular at an interface;
• a hollow piston slideably moveable within an inner bore of the body, dynamic seals being between the piston and the inner bore's inner surface;
• the piston having features, including one or more ledges, holes, or grooves, into which a détente may extend or attach;
• the détente, being field serviceable without significant disassembly of the apparatus, for holding the piston in place until pressure on one side of the piston exceeds a threshold pressure;
• the piston being moveable when the détente is released to slide within a cylinder formed by the inner bore;
• and
• a disc, being breakable and having a circular edge with a disc ledge and a domed surface, the disc ledge coupled to one side of the piston, at least a portion of the disc being exposed to the inner bore, the disc dividing the inner bore into two volumes, each volume in fluid communication with the wellbore tubular's inner conduit;
• the disc being exposed on each of its sides to fluid pressures exerted by fluid in the wellbore tubular at the interface.