Try all your crankcase bolts, mine leaked like that and I found some of the bolts slack.
What brakes do you have presently, mine are pretty good, I have alloy brake plates with heavy duty cables and rear brake arms on the front, these are about 50% longer and help a lot.
I have a home made 8" TLS brake in my 11-50 Brough that uses Commando shoes and it has always been poor, I recently had the shoes relined by Saftek with their soft, high friction material and the difference is incredible, a real brake, and I mean a real brake, and that was without turning the shoes as they were fractionally undersize anyway.
It's a woven green/yellow sintered material.