Description and a Flash movie of the booster in action:
Hissing at rest means either:
- The seal between the housing and the piston assembly has gone bad.
- The internal valve isn't seating properly, allowing outside air to constantly
leak through the booster. Could be a piece of dirt, a chunk of the air filter
(the pedal's pushrod goes through a foamy, spongy thing... the filter), or
some kind of corrosion on the valve's seat keeping it from closing.
Pressing the pedal a bit moves the internal valve enough to seal the front
chamber from the rear, so no more flow.
When mine started hissing years ago, by the time I got it to the shop later
that day (maybe a 20 minute drive), I had to stand on the pedal with both feet
to stop, so yes, it IS a critical safety issue, unless you don't care about your
life or that of others around you.
You problem can progress to a sudden catastrophic failure
very quickly without further notice. It is a safety concern
and should be repaired at the earliest opportunity with as
little vehicle operation as possible until it is repaired.
The ass you save may be mine!
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