The brief
The A4's brake pedal had gone hard, you had to push much harder than usual to slow the car, with a brake warning on the dash. He brought it in straight away, the right call, that's a safety system and a hard pedal means the assistance has dropped out. That points at the brake booster actuator. On this Audi the brake assistance is electromechanical, an actuator that multiplies the force you put into the pedal so the brakes bite without you having to stamp. When that actuator fails, you lose the boost, so the pedal goes stiff and the braking effort goes up, and the management lights the dash. A failed brake booster actuator doesn't recover, so it needs replacing, and the brake system bled and the unit coded to the car afterwards.
The diagnosis
A diagnostic scan pulled the fault to the brake booster actuator, the assistance not coming through, which is exactly the hard pedal and the warning. The brake pads, discs, calipers and the hydraulic side checked out, it was the actuator at fault. That's a brake booster actuator replacement, you don't rebuild it on the car, so the call was a complete unit, fitted, the brake system bled, and the actuator coded to the car.
The work
The old brake booster actuator was removed and a new genuine Audi-spec unit fitted, connected up, every fastener torqued to spec. The brake system was bled through with fresh fluid until it ran clean and the pedal was firm, the new actuator coded to the car, and the stored faults cleared. A road test confirmed a normal pedal with proper assistance, even progressive braking, and the warning light off.
The outcome
A normal brake pedal with the assistance back, even progressive braking, no hard-pedal effort, and no warning light. The A4 went home with the braking sorted. A failed brake booster actuator leaves you fighting the pedal, so changing it, bleeding the system and coding it in put the braking back where it should be, a job you don't put off.
