Braking effectiveness is reduced going backwards, no question about that.
I think the main reason is contact - when the entire shoe assembly rotates in the reverse rotation, the shoes don't totally contact the drums. Not just on this one, but ALL Bendix drum brake systems.
Since we no longer use a shoe arc grinder to get the shoe to the same arc as the drum, putting on new shoes they only contact partially. They have to be "broken in" or "worn in" till they contact better. That's why the first 100 miles or so is important not to overheat them. But that's in the forward direction.
I'm sure everyone who has done brake jobs has seen linings that were not worn on the entire surface - the arc was not matched.
Replacement shoes are usually factory arced to about .040 oversize, they assume the drums have been turned. (Ever put on "replacement shoes" with a new drum? The shoes barely fit with the star all the way down.) In the "old days" we arced the shoes to the exact size of the drum - they worked so much better then. (Thanks Tree Huggers, we don't need brakes that stop anyway - please step in front of me.)
Even if the forward direction had worn in properly, when the shoe assembly rotates in reverse contact is from a different angle. The linings were "broken in" to the forward position, but now that the assembly is in a different position, contact area is greatly reduced. The shoes still contact, but not along the entire surface, just in one or two spots. That greatly reduces braking effectiveness.
Yes, since the contact area is smaller, it will build up heat faster - if it's used for a long time.
But - in this case - he applies the brakes AFTER storming the hill. While he was storming up, brakes were not applied, no heat there. The only heat present would be from the last hill climb - if he even used the brakes much it had time too cool.
The other possibility of heat would be from the exhaust - which of course is very hot from the "storming" up the hill. If the brake lines had been moved too close to the exhaust, brake "fade?" would be experienced in both directions, not just reverse.
Same way for water in the fluid. (How does it "know?")
If he's hill climbing in a competitive event - he'd have a fairly long wait between runs, letting the brakes get very cool.
Or if he's just playing on hills, like I do, he's using the brakes more frequently but not near enough to experience heat fade, no matter bad the brakes are.
And - he mentioned the pedal sinks at the same time - heat fade, at least with the heat fade I've experienced, the pedal did not go down, it stayed up, but the brakes just didn't stop like they should. Pumping or pulsing the pedal didn't help - it was overheated linings that caused it.
But - improperly adjusted brakes, including the parking brake, will do it every time!
Try it - loosen your brakes with the star wheel, after about 15 clicks each wheel, step on the pedal - first time goes low, subsequent pumps get it back up.
Drive it - notice if you apply the parking brake a little you feel the pedal get higher for a moment (you are biasing it off the normal "rest position.")
Keep giving it a little more parking brake until it drags all the time, then back it off slightly. Now you have duplicated Patrick's.
Now - back up and hit the brakes - make sure you have nothing behind you, NO BRAKES! Pump them a few times, they work again.
No heat involved here!
BTW - that's a diagnostic trick when diagnosing brake problems - without ever pulling a drum, you can tell alot with how the parking brake reacts.
But of course, - for those that "Don't want it to be that," it isn't.