I am not familiar with sherwood, but according to Harlow, they're 2nd are all classic downstream designs. The typical schematic he includes show an adjustabe orifice. He also cautions to be careful with Sherwoods as they have a problem with cases cracking. If you remove the hose and look into the opening you should see orifice. It is the restriction with the hole in the middle and should be slotted so it can be turned with a screwdriver (some use an allen wrench if not slotted). You would need a fairly wide bladed screwdriver to engage the slots. It should turn fairly easily. You don't want to scratch anything. Depress the purge button when you make any adjustments so the seat is pulled away from the knife edged orifice which can cause cutting. Adjustments shoud be make about 1/12 turn at a time (30 degrees). Turning the orifice counter-clockwise reduces the contact with the seat and results in easier breathing or a steady leak. Clockwise makes breathing a little harder or stops the leak if you have backed off too much. If it is breathing OK but a little hard now, I would expect 1 or 2 adjustments (1/12 turns) to correct the problem and 2 or 3 should start it leaking. If 1 or 2 turns does not make a noticable difference then the problem may be more complex than a simple adjustment.
If that is more than you want to try, a shop should do this for you in about 5 minute (10 if he includes a good inspection) for under $20.
