Whilst taxying for departure, the pilot of VH-CLA noticed that the left brake was becoming progressively less effective. He attempted to slow the aircraft by closing the throttles and using the right brake with full left rudder application. However, this proved ineffective. As directional control was becoming difficult the pilot elected to turn the aircraft to the right onto the grassed area beside the taxiway. He applied right brake but the aircraft turned rapidly through about 165 degrees and collided with VH-ILS which was following behind. The pilot of VH-ILS reported he had lost sight of VH-CLA immediately prior to the collision due to condensation on the inside surface of the windscreen, and from watching another nearby aircraft. Investigation later determined that the left brake piston seal was in poor condition and leaking, and the master cylinder was almost empty. There was an outstanding entry on the Maintenance Release to bleed the right brake.