No brakes on an 8100 Chevrolet when the engine dies?

By 8100 I am guessing that you are referring to an 8.1L engine? To answer the question though your braking system is a power assisted system. Depending on application engine vacuum acts on a diaphragm in the brake boost canister helping to apply power to the actuation rod that is pushing on your master cylinder piston. The other application is a hydro boost set up where Power Steering fluid is routed to an accumulator and is used in much the same way. The brake pedal is pressed and a rod presses on the master cylinder piston, this opens a valve allowing pressure stored in the accumulator to act on the piston. In either application, when the engine dies there is no longer a supply of vacuum or power steering fluid to create pressure for the power assist. It feels like you have no brakes but you still do. Just press harder on the pedal.