Dear Mr. Weekend,
Hope you're well. It's possible that the brake booster's check valve has failed. This valve is a round black (or white) plastic unit (Volvo Part #270596, if your 960 is a '95 model) mounted in a black rubber grommet at the "ten o'clock" position.
This check valve - Bosch part No. 0204120018 - can be found at: https://www.amazon.com/Bosch-Power-Brake-Booster-0204120018/dp/B00757N5BC .
The booster units seem to be pretty durable. As the check valve is relatively cheap, accessible, and easy to install, it might be worthwhile to change it, to see if that helps matters.
Hope this helps.
Yours faithfully,
Spook
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