As Kev says, If it feel good on the road and does not creep down all the way to the floor it is normal.
Leave it running for a a few minutes with foot off, switch engine off then press the brake, if it dose not creep or touch floor it is fine.
A few years ago there was a special MOT notice about it and wording in the manual amended for 'high vacuum systems'
It becomes noticeable on diesels as the Vac pump is relatively slow in operation particularly to as it get to the high end of its range.
Aidan
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