71 degree slope without either slipping or falling over forwards.
useless analogy.
there are more than brakes involved, tyres and technique are imo more important. what tyre pressure is the op’s fella running and what tyres. I’m always shocked by the high pressure that people run on there bikes, and how pants this makes braking power. if he’s running more than 30psi he needs to let them down.
the trick to gaining more braking power is to gain more traction so that you can use power that would normally lock up the wheels, and to throw the bodyweight backwards so that power that would normally throw you off the front of the bike will stop you.
if the brakes don’t lock up on gravel/ tarmac then somethings wrong.