A technical explanation of how aircraft brakes work would cause me to write for roughly 50 pages. Aircraft operations manuals usually have an entire chapter devoted to these systems. Braking systems on airliners are hydraulic, often involving complex systems of pressure valves in the case of extended parking brakes. The parking brake is not really different than the toe brakes. In some airliners, you set the parking brake by holding down both toe brake pedals and then activating a lock switch. This switch, located in various spots, (depends on model) locks down the two pedals much like the type of E-brake that some cars have. Although I've never flown any of the new ERJ's, I noticed they have a parking brake lever similar to that of many stick shift model cars. Whether it be a locking toe brake or a seperate lever, there isn't a seperate parking brake. Most airline regulation requires that these parking brakes hold the aircraft in position for 18 hours, 24 in some cases. Problems arise sometimes because hydraulic pressure brakes have a constantly adjusting valve that relieves pressure resulting from heat/cold. If enough pressure is released, the plane will go for a roll. This is why you also see blocks behind the wheels of parked aircraft.