The symptoms describe what would happen if you hadn't backed off the clutch adjustment far enough for the clutch to actually free. Bear in mind that you have a new belt (which is relatively stiff) and an idler pulley clutch system that doesn't free very well at the best of times. If more slack in that pullrod stops it from creeping, then more slack is what you need. The brake is not there to hold the mower against pull from a tight clutch adjustment: it is there to keep the mower from rolling downhill.
