purge hash table, how to?

i am using postgresql 8.0.3 as a single user by running "postgres"

I want to purge all contents in the bufferpool, and I did this by
calling "InitBufTable(256)" (buf_table.c)  after each query. However,
this seems not working for each followup query I still get less disk read
(tracked by "smgrread()"  in smgr.c) and increased bufferhitcount.

Could anyone tell me which module shall i modify instead?