Werner,
The behavior you describe is typical of a brake servo not getting
enough vacuum at idle. Engines with large cams or with emission
controls don't generate a lot of vacuum at idle and, unless aided, can
loose brake boost over time.
Check the vacuum check valve located in the booster vacuum supply line.
If this is working properly and the servo holds vacuum, the pedal
should not rise.
If everything checks out and you still have the issue, add a vacuum
reservoir tank. This will store "vacuum" developed during cruising for
times of need like idling. Many modern cars have these so finding one
in a junkyard should be no problem.
Good luck,
Bill Eastman
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