The air vavle washer is the first item I would replace out of the assembly (I think they are sold by the dozen). I understood from your post that you had most of the air valve assembly out? there is an air valve nut just inside the part where the air hoses attaches to the brush. Then there is the spring, valve and the washer. Try using a pin to get the washer out, or a piece of bent wire (maybe a bent paper clip or something?).
If you have free needle action then maybe the packing washer may not need replacing but if it were me I would want to take a look at it.
I only suggested that I would buy the metal handle if I was ordering parts simply out of vanity sake. There is nothing wrong with the plastic handle. I would tend towards the full length handle over the stubby but I really would have to try the brush with both to make an learned opinion. I still have the plastic handle on my brush and I have had it over 10 years which goes to show you how important it really is to me to have the metal handle
Just one of those "would be nice to have" things.