Car lurches forward when braking?

It's not your timing belt. The timing belt would never cause this. The power brake booster; which is located between the firewall and the master cylinder, has a hose that goes from the power brake booster to the intake manifold. The engine provides vacuum to the power brake system to assist you in stopping the vehicle. The booster magnifies your efforts, without it, it would be much harder to press on the brake. The booster has failed and is now pushing air into the intake. The computer has to compensate and provide more fuel to keep the air/fuel mixture correctly. You should have the booster checked out and replaced.