I got a Vac meter and the pump seems strong and fine, 25 right away.
There is a slight hissing sound when stepping on the breaks, the pressure goes to 0 when pressing the breaks the first 1'. But when I hold the breaks down the pressure goes up again to 25. When releasing the breaks I loose some pressure again.

Any chance it's not the booster?

My manual did not have any info on changing it, just the master break cylinder... any ideas?

thx