Join Date: Aug 1998
Location: Waldorf Md. USA
The "hissing" sounds like a vacuum leak. The fact that the problem occurs when you have your foot on the brake is another clue, the power brakes work with vacuum. I would do some investigating in that area. Could even be a bad booster.
Hope this helps,
Advanced Automotive Machine
1971 Chevelle "Heavy Chevy" original owner
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