Cause
Failed vacuum check valve
The one-way valve that holds vacuum in a circuit after the source stops making it, no longer sealing. It is a small, cheap part doing a job that is easy to overlook: not creating vacuum, but keeping it.
That storage function is what makes it matter on modern vehicles. A turbocharged petrol engine makes almost no manifold vacuum at part throttle, a diesel makes none by design, and a stop-start car has the engine off at every junction — so the vacuum for the brake servo is banked rather than continuously supplied. A valve that leaks back empties the reservoir between uses, and the symptom appears at the worst moment: the first brake application after the engine has been off.
The classic report is a pedal that is hard on the first press and normal afterwards, which is the servo being refilled by an engine that is now running.
They fail from age rather than abuse. The rubber flap or the diaphragm hardens, and a valve that has been baked next to an engine for ten years is at the end of its life whether or not anything else is wrong.
Check its orientation after any repair. These are directional and will fit either way round, and one reversed produces a fault indistinguishable from a failed pump — for a part that costs almost nothing.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (6)
Codes where this is a likely explanation first, rather than merely a possible one.
- P2525 Vacuum Reservoir Pressure Sensor Circuit Likely for this code High
- P2526 Vacuum Reservoir Pressure Sensor Circuit Range/Performance Likely for this code High
- P2528 Vacuum Reservoir Pressure Sensor Circuit High Likely for this code High
- P2529 Vacuum Reservoir Pressure Sensor Circuit Intermittent Likely for this code High
- P258B Vacuum Pump Control Circuit A Range/Performance Likely for this code High
- P0556 Brake Booster Pressure Sensor Circuit Range/Performance High
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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