Testing on a bench or with an MoT doesnt adress the issue the OP raised. The problem with drum brakes is dissipation of the heat that is generated when you brake. You can help this a bit by replacing cast iron drums with Alfin type aluminium drums as was common in the 60s but the real cure is disc brakes as jaguar demonstrated at le Mans. The disc and piston is exposed to the air and is way better cooled than a drum.
If the brakes get really hot then water in the brake fluid boils and maybe even the fluid itself boils. This means gas in the brake lines which is easily compressed unlike a liquid and so the pressure on the brake shoes is reduced and the braking effect is reduced too.
