Some Volvos have used a PCV valve of 1960s design that has been called an anti-backfire valve. When it fails the idle will be affected and oil might leak to the back of the engine and out the dipstick tube and the brake pedal might become more difficult to press. The valve is connected to what was long ago called the breather box as it was an oil separator with a road vent tube attached. But I am reading between the lines here FWIW.