The operator was conducting an aerial spraying operation from a private airstrip, and had completed three loads, earlier in the morning. After applying the fourth load, the pilot returned to the airstrip for another load. During the landing the pilot applied both brakes; however, there was no response from the right-side brake. The pilot reported that he pumped the brake several times, but still no response. The pilot then applied full left brake and the airplane spun 180-degrees, collapsing the right (main) landing gear with the propeller striking the ground. The airplane impacted a ditch and electrical pole, before coming to rest. The reason for the loss of brake pressure was not determined