I noticed most people state hissing occurs when they push on brake but mine happens after I push the peddle down and its rising. Does that still sound like a bad booster or could it be the check valve in the hose instead. Also, the pedal is not difficult to press. Thanks in advance for replies.

 

If its soft and hissing then it is the booster, common issue on Volvos 2001+, ATE makes this junk. There is a good write up on how to replace it.