1975 Commando front brake MC

My 1975 Commando has been allowed to sit for 8 years. The front MC was re-sleeved by Old Brits. The piston was locked in the cylinder. A gentleman at an old British bike shop was able to remove the piston and told me that the MC would most likely leak if the original piston was reinstalled. I have typical problems of a 79-year-old and would like to put the old piston in. Is there a site that shows a diagram showing how the parts are put together?
Thanks

$pend on a new 13 mm Master cylinder . Andover Norton .

Sleeved master cylinder sellers fooled me twice; I would never use a sleeved MC for anything but a paper weight.

I had two re-sleeved Norton MCs fail. Both were from a well-known supplier of sleeved Norton MCs. One failed overnight in the garage, the second failed a year or two later between stoplights in town - worked perfectly at stoplight #1, no brake/lever went to bar at stoplight #2.