Well, with the engine off, you have no vacuum assist on the brake booster. So you are feeling the unassisted master cylinder. Not sure if you have ABS or not, but one thing you could try for a test is to plug the vacuum line to the booster and then start the engine and see if anything changes with the engine running (and no brake boost). If nothing changes, it could be the booster. Or if you have a vacuum pump, try pulling a vacuum on the booster w/ engine off and see if it does the same thing as with the engine running (and booster connected).

r.c.brown@ieee.org
TruckEditor@tlca.org
Project: 4Crawler
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