Tcpdump writes 0 bytes

[duke.tim]

At work we have setup a sniffer, on a Cent-OS machine. It seems to caputure and write pcap files fine.However if tcpdump is ran on the pcap file and set to write a new filtered pcap, it writes a 0 byte file.The thing that is strange is if tcpdump is to filter without writing the file, stdout shows up fine.
tcpdump -r /pcap/location port 80 Sucess
tcpdump -r /pcap/location -w /newpcap/location port 80 Writes a 0 byte file
and
tcpdump -i eth0 -w /pcap/location suceeds
Any ideas on what could be wrong?

[duke.tim]

Well I managed to figure what the problem was. As it turns out, tcpdump was using the wrong permissions. Needed to use the -Z option!