Iron Horse,
Older machines I have worked on had an emergency brake type lever that operated the swing parking brake. Others were a valve that operated a cylinder that pushed a lever that tighted the brake. It was all kind of a dinosaur thing but they worked to some extent.
Haul Pak,
You are pretty close to how it works but there are a couple of different things going on. There are relief valves on the motor, one for each direction of flow, that are set at about two thirds the system pressure of the machine. They each relieve into the up stream side of the other valve. Most people call them cross over reliefs but I have also heard them called safety suction valves. Because there is so much reduction in the gear reducer below the motor, the torque can be applied both ways, starting to swing and stopping the swing. When the swing gets mushy its usually because the seats on the cross overs are going bad and starting to leak through at a lower pressure.
The closed loop system you are speaking of uses a dedicated pump that will pump oil in two directions. The pump is usually controlled by some sort of hydraulic actuator, I have seen cylinders and mechanical linkages in old machines and servo pistons in newer machines. This pump feeds a fixed displacement by-directional motor. Pump oil into one side and the house turns one way and simply reverse the oil flow and the house goes the other way. There are suction checks in this system so that when you let go of the control lever the oil will freely go around the loop. Koehring excavators used to use this system and I'm sure lots of really big machines still use it.
scholzee,
Excavators use a swivel joint by one name or rotary manifold by another name. Basically it is a tube that is stationary with the either the house or carbody and a center section, that is stationary with the other. The center section is like bar stock with what look like thick plates that fit into the tube. The outside ends of the plates have cylinder seals. Tubes connected on the outside of the barrel correspond to the spaces in between the plates.
Once you see one you will think how simple it is.