cannot unmount nfs version 3 export on rhel/centos 5

We have a server exporting a filesystem with nfs version 3. rhel5 clients cannot unmount the filesystem and have this error:

[root@client /]# umount /var/spool/mail
umount.nfs: server.example.com:/export: not found / mounted or server not reachable

Running the umount with -v shows the problem.

[root@client /]# umount -v /var/mail
mount: trying 192.168.0.1 prog 100005 vers 1 prot tcp port 4002
umount.nfs: server.example.com:/export: not found / mounted or server not reachable
mount: trying 192.168.0.1 prog 100005 vers 1 prot tcp port 4002
umount.nfs: server.example.com:/export: not found / mounted or server not reachable

The client is trying to talk version 1 with the version 3 server. For some reason the ount steps up to version 3, the umount doesn't. The workaround is to specify vers=3 in the mount line.

[root@client /]# cat /etc/fstab |grep server
server.example.com:/export	/export	nfs	vers=3,rw,nosuid,noac,rsize=32768,wsize=32768	0	0
[root@client /]# mount |grep server
server.example.com:/export on /export type nfs (rw,nosuid,remount,nfsvers=3,noac,rsize=32768,wsize=32768,addr=192.168.0.1)

After that the umount works fine.

[root@client /]# umount -v /export
mount: trying 192.168.0.1 prog 100005 vers 3 prot tcp port 4002
server.example.com:/export umounted