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 .