Something is definitely wrong - the brakes on these vehicles are usually magnificent.
There should be a one-way valve in the vacuum line - perhaps that is faulty.?

I spent some time looking for a replacement booster - then found one on eBay UK for 20 pounds.! Seller had good feedback and was a dismantler of European cars.
I had it freighted to our side of the planet and it works perfectly.

My Volvo is simply awesome.

So I bought another...

http://www.kilroy.co.nz/images/car35_3.jpg