Brake booster noise or what?
 

Driving last night noticed a hissing sound from under the dash. Isolated it to the brake booster. The brakes work'great. The pedal is firm with good feed back. The pedal doesnt drop to the floor there is no vacuum leak, no idle issues.The pedal firms up and stays up when pumped. I am thinking that the brake booster is leaking because of a bad diaphram. Replacement oem about $950. oem from overseas supplier about $700 with shipping , sieken brake booster 290 with shipping or the cheapest option oem junkyard.
2000 4runner 400k+ miles
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So this is the sound they make before they fail, guessing you read that, not sure, I mean if brakes work otherwise OK. Super loud or just the video?

Last night driving down the montain long 7% grade in 3rd utilizing engine braking was'loud enough to hear it over low radio noise and the motor. Today in the foothills not as loud. Just concerned with two little ones have to go backup next weekend and don't want to take any chances driving the front range.