Got the booster back together. The spring pressure is not very hard to overcome, perhaps 40-60kg pressure required. Compressing the new rubber gasket however, requires quite a bit of force.
But as they say, it's when the going gets weird, the weird get going.
I could probably have used the hydraulic press at work, but this was a far more interesting approach.
Brake master cylinder dismantled.
This is where the progress yet again haulted. The repair kit I ordered from "Superperformance" for a Ferrari 308 does not fit. The bore in the Deauville is 1" (25,4mm), while the ferrari parts seem to be 22-23mm.
It seems that the GTB/S has a 1" booster, but so far I've been unable to find seals for one of those.
The used booster for a Ferrari 308 came with a master cylinder, which also has a 1" bore, but the internals are completly different from those in the Deauville. I'll take some comparison photos when I have an opportunity.