* Master Cylinder Issues: A failing master cylinder might not be providing enough hydraulic pressure to the calipers or wheel cylinders, even with fresh fluid. Internal leaks or a failing piston seal are common culprits.
* Wheel Cylinder/Caliper Problems: While you replaced the brake pads, the wheel cylinders (rear drums) or calipers (front discs) themselves might be worn, leaking internally, or sticking. This prevents proper piston movement and brake pad engagement. Seized calipers are a common problem on older trucks.
* Proportioning Valve Issues: The proportioning valve controls the braking force between the front and rear brakes. A malfunctioning valve can lead to unequal braking, making stopping difficult and potentially dangerous.
* Brake Lines/Hoses: Internal rust or corrosion in brake lines or hoses can restrict fluid flow. Even a small leak will drastically reduce braking power. These are particularly vulnerable in older vehicles.
* Vacuum Booster Issues: The vacuum booster assists the master cylinder. A leak in the booster, a malfunctioning check valve, or a vacuum leak from the engine will result in weak braking. A failing booster will make the brake pedal very hard to push.
* Bleeding Issues: Air might still be trapped in the system despite your attempts to bleed the brakes. This is a common problem if done improperly or if there are leaks elsewhere. Some bleeding procedures might not be effective for certain master cylinder designs.
* Parking Brake: A partially engaged parking brake will also affect stopping power.
* Brake Rotors/Drums: Severely worn or warped brake rotors (front discs) or drums (rear drums) will reduce braking effectiveness, even with new pads.
* Low Fluid Level (despite bleeding): You may have a leak somewhere in the system that is causing a loss of brake fluid, even if you filled it up after bleeding the brakes.
In short, while replacing brake pads and bleeding the brakes addresses *some* potential stopping problems, it doesn't solve *all* of them. A thorough inspection of the entire braking system, from the master cylinder to the calipers/wheel cylinders, is necessary to diagnose the underlying cause of the braking problem. If you're uncomfortable with this, a qualified mechanic should be consulted.
