A rear brake that locks with almost no warning is a genuine hazard: on a wet Vietnamese road, a locked wheel skids instantly, and skids fall over. Drum brakes — still the standard rear brake on most local bikes — are simple, and their locking causes are all simple too.
The difference between bite and lock
A healthy drum brake is progressive: pressure rises, braking rises. A brake that goes from nothing to locked within a few millimetres of pedal travel has lost its modulation, and modulation is the entire game. Causes, in order of frequency:
Worn shoes and the self-adjuster story
Drum shoes wear gradually, and many drums rely on adjusters — automatic or a simple threaded rod — to keep the bite point near the top of the pedal. Worn shoes past their limit, or a corroded, lazy self-adjuster, leave a long pedal… until the cam suddenly goes over centre and the shoe jams hard against the drum: that is the sudden lock. Check the pedal free play per the manual (part of brake-checks-motorbike-vietnam) and have the shoes measured when the drum is open; shoe thickness limits are a manual figure, not an estimate.
Corroded cam and return springs
The actuating cam — the little eccentric that spreads the shoes — corrodes in humid air and after flooding. A rusty cam sticks in the applied position: the brake drags, heats, then locks on its own at the next application. Weak or broken return springs produce the same refusal to release. Inside a drum, this is a strip-and-clean job: clean the cam, grease only the specified points sparingly (grease on friction surfaces ruins braking), replace tired springs.
Cable and linkage drag
Rear drum brakes are usually cable or rod operated, and a cable that drags inside a rusted sleeve holds the brake partly applied: the drum heats, the drum expands with heat against the shoes, and the cycle ends in a lock-up, usually in traffic, usually hot. A fully free pedal return — snapping back on its own — is the test; if not, clean and lubricate the cable or replace it. The dragging-brake logic in general is in sticking-brakes-caliper-cleaning.
Water and the wet-drum moment
After deep puddles or flooding, a wet drum loses friction, then recovers unevenly as it dries: an application early in a ride can be weak, and a minute later, grabby. Riding the first few wet applications gently, testing braking before you need it, is the discipline; the wet-road risk picture is in after-rain-slippery-roads-motorbike.
The lock itself
If the rear locks while riding: look up, keep the bars straight, ease off the brake smoothly, and let the wheel spin back up. Slamming the front brake while the rear skids converts one problem into a crash. Practice controlled braking in a safe empty area — the technique of braking at the limit belongs to the riding-safety guides, but the machine-side fix is what this article owes you: a brake that locks predictably at the same pedal position, every time.
For the disc-brake equivalents of these faults and the drum-vs-disc trade-offs, see disc-vs-drum-brakes-motorbike.