What evidence do we have that Chief Bromden cannot be entirely trusted? What do we still trust in what he says? please put clearly respond

This is the story
Black boys in white suits up before me to commit sex acts in the hall and get it mopped up before I can catch them.
They’re mopping when I come out the dorm, all three of them sulky and hating everything, the time of day, the place they’re at here, the people they got to work around. When they hate like this, better if they don’t see me. I creep along the wall quiet as dust in my canvas shoes, but they got special sensitive equipment detects my fear and they all look up, all three at once, eyes glittering out of the black faces like the hard glitter of radio tubes out of the back of an old radio.
“Here’s the Chief. The soo-pah Chief, fellas. Ol’ Chief Broom. Here you go, Chief Broom ...”
Stick a mop in my hand and motion to the spot they aim for me to clean today, and I go. One swats the backs of my legs with a broom handle to hurry me past.
“Haw, you look at ’im shag it? Big enough to eat apples off my head an’ he mine me like a baby.”
They laugh and then I hear them mumbling behind me, heads close together. Hum of black machinery, humming hate and death and other hospital secrets. They don’t bother not talking out loud about their hate secrets when I’m nearby because they think I’m deaf and dumb. Everybody thinks so. I’m cagey enough to fool them that much. If my being half Indian ever helped me in any way in this dirty life, it helped me being cagey, helped me all these years.