There need not be excessive positive crankcase pressure. It just happens that there is vacuum at the manifold end. Possible reasons are that there is not excessive pressure and 1) faulty PCV 2) hoses old, hard (or soft) and leaky 3) flame trap plugged 4) wrong nipple feeding PCV (i.e. non-reduced fitting and hoses switched from brake booster and PCV). There could be crankcase pressure too - it is a 41 year old car but I would lean more toward the idea that something else is wrong and the vacuum will simply draw air if the rest of the ventilation system is not in good order. I think somebody else mentioned that Volvo's crankcase ventilation system changed over the years - you have to figure out what your year was supposed to be and then make sure it is routed correctly.