patching a deb package using "ar" for unpacking and packing

I have a deb package provided by an organization. I found that there was bugs in the code(python). Thus,

1. I unpack it with "ar xv abc.deb"
2. unpack tar.gz with "tar zxvf control.tar.gz data.tar.gz"
3. fix bugs
4. update md5sum
5. packing control.tar.gz with "tar zcvf control md5sum postinst prerm"
6. packin开发者_开发问答g data.tar.gz with "tar zcvf data.tar.gz usr"
7. make deb with "ar rcv abc_patch.deb debian-binary control.tar.gz data.tar.gz"
8. put abc_patch.deb in my own ppa

However, I then apt-get install abc_patch.deb but it raised "size mismatch".

Did I make any mistake in patching the deb package ?

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Update the minor version in the control file?