The pilot made a short local flight and in very light wind conditions returned to land on runway 23. During the landing roll the aircraft slowed and as the tail was lowered a swing to the right commenced. The pilot applied maximum left brake but was unable to prevent the aircraft leaving the runway at an angle of about 30 degrees and then ground looping right. The left main gear leg collapsed. The pilot reported that he believed that the right brake locked on initial application. Ground marks indicate that both brakes were locked when the aircraft ran off the sealed runway. The aircraft is not equipped with a steerable tailwheel and with the flaps lowered for landing, as on this approach, the rudder is ineffective once the tail is lowered.