Spring, rear wheel brake disc return. Tractors: 3550, 4000 (except 4110), 5000, 7000 (1965-1975), 3900, 4100, 4600, 5600, 5700, 6600, 6700, 7600, 7700, 420, 515, 532, 535 (1975-1981), 2810, 2910, 3910, 4110, 4610, 5610, 5900, 6610, 6710, 7610, 7710, 7810, 7910, 8210 (1981-10/1991), 3230, 3430, 3930, 4130, 4630 (3/1990 & up), 5640, 6640, 7740, 7840, 8330 (with mechanical brakes) (11/1991 & up), 5610S, 6610S, 7610S, 7810S (1/1994 & up).