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SO.... I had to strip some models from the pain awhile ago, so, I soaked them in brake fluid like I always do.
There was one finecast model there aswell, a Tyranid Zoanthrope. I left them to soak for 3 days in total and other models were just fine but the zoanthrope was all rubbery and broke easily, all thou' I managed to fix the parts that broke but still... will it return to its old state if I wait? Is there any way to restore it?

Bummer! I suspect the brake fluid has leached out some component of the plastic resin and it may never be the same again. I had a similar thing when I soaked plastic Termagants in acetone based fluid for too long.

However, you should at least wait a few days to see if it will strenghten up.

If that fails, I wonder if it may be possible to use some other kind of plastic solution to soak the resin and infiltrate a strengthening component back into it. Maybe acrylic floor polish or acrylic media.

I'm guessing out loud here, so don't rush off and try it before some other people have given their advice.

Resin and brake fluid are not good friends.

I use brake-fluid to strip my models all the time, but I only let them soak for 10-12 hours - in the case of resin even shorter.
I have found that the resin sort of "absorbs" part of the brake-fluid. Last time I had to let the resin part dry for about a week before it stopped "sweating".