شیر یک‌طرفه خلأ خراب

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.