Staale,
Chaitanya could be right. Although I have not had any problem with the 6 PST air traps I have used so far, I have heard that some had a problem with the brass porous pickup clunk.
Have a look here on how I check/modify the PST air trap for the P-200:
http://www.rcuniverse.com/forum/fb.asp?m=9297018
I still recommend you to do the flow test...