An interesting test would be to measure the voltage at the contact with the bulb installed, the socket grounded and the brake pedal pushed. I'm betting you'll find very little voltage.
If so, the problem is upstream resistance as 3B suggested earler.
'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.
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