If the reflective surface of the sealed beam lights deteriorates, most of the light is lost out the back of the bulbs. I've seen this happen several times. Foe some reason, moisture can penetrate the lights, and cause the silvery coating of the reflector to degrade. Just for a check, try a new sealed beam, and see what happens.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!