This drawing I did years ago when troubleshooting my turn signals may help. It was from my '57 GMC, but all early GM turn signals (before 4 way flashing lights) that used the same rear light filaments for brake and turn signals worked the same way. Ignore the wire colors as yours may be different.
The top drawing shows switch connections when the turn signal lever is "off", the center drawing is with the lever in the right turn position and the bottom drawing is with the lever in the left turn position. All 3 yellow contact sets move at the same time and direction inside the switch.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
