When I purchased the car it had the single reservoir master cylinder. A good safety upgrade was to replace this with a dual reservoir master cylinder so that if a brake line did go I would still have some brakes!
I wasn’t as confident with the car as I am now (5 years of experience…) so I paid a local speed shop to do it for me (as well as installing new rear leaf springs. That was only 5 years ago but I recently noticed that the fluid level in the rear reservoir was low. After topping it up I found it was low again within a few days. No obvious leaks at the wheel end but I found a rather major leak right where the cylinder meets the firewall. It seems a seal had gone and although you can get repair kits, it was better to replace the entire thing, especially as the entire thing had turned a nasty rust colour (Not the best weather in Ireland for keeping a car outside!).
My Mustang Journey 