Vacuum Build-up Time for BKV Systems

Hello fellow enthusiasts,

After a more extensive brake modification (ABS -> ABS/EDS, please don't discuss the reasoning behind this here...), I noticed during the final test that, with the engine running (idle) and the brake pedal fully depressed, the pedal slowly sinks.

That's not normal, is it?

A vacuum measurement showed that it takes approximately 20 seconds for the vacuum to reach about 700 mmHg (final value).
After the engine is turned off, the vacuum drops to half its value within approximately 4 minutes. I measured this on the pump side, i.e., before the check valve.

Unfortunately, the repair manual does not provide any information on the absolute values or 'half-life' of the vacuum pressure.
A search in the forum (under the topic 'vacuum brake') also yielded no clues.

Are the values mentioned above within the acceptable range, or does the vacuum pump need to be replaced (for €290!)?

I look forward to your replies.

Best regards, K. Oldewurtel.