HOW to clear Buffer cache in linux ?

1)To check buffer size.
# top
top - 10:03:51 up 3 days, 20:03,  3 users,  load average: 0.53, 0.78, 0.83
Tasks: 487 total,   1 running, 486 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.7%us,  0.4%sy,  0.0%ni, 99.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:  132350512k total, 119378720k used, 12971792k free,  1817552k buffers
Swap: 157286396k total,       20k used, 157286376k free, 110546064k cached
2) to release buffer size.
# sync && echo 3 > /proc/sys/vm/drop_caches

3)to check buffer size.
# top
top - 10:15:41 up 3 days, 20:15,  3 users,  load average: 0.73, 0.76, 0.80
Tasks: 485 total,   1 running, 484 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.7%us,  0.3%sy,  0.0%ni, 99.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:  132350512k total,  2502388k used, 129848124k free,    13256k buffers
Swap: 157286396k total,       20k used, 157286376k free,   137032k cached