Keep your Dad away from your bike! What a disaster..
The dust seals are important because they also retract the piston slightly when you release the brake. Without them your brakes will rub and overheat. I recently used pattern seals & pistons from Bitzforbikes and I reckon they are fine.
Fitted a thou caliper, posted up a pic of the finished fit and one of a banjo sitting above the bit you need to file.
I did it while the caliper mount was on the bike. I tucked a cloth between the mount and the discs so I didn't scratch the disc.
Andy-just make sure the caliper or mounting plate doesn't foul brake line under movement of swing arm etc. BUT YOU KNEW THAT ANYWAY :eek
Glad you sorted problem. Go easy on the pedal! :lol