Date registered: Jan 2004
Vehicle: W-1-2-3 Go!
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When the car is running and you pump the brakes, the booster uses up whatever available vacuum assist the vacuum pump (on a diesel) can provide. So yes, the brakes get stiffer. Then when you stop pumping, with foot firmly on the brake pedal, the pedal should lower a little when available vacuum is sufficient to provide vacuum or booster assist.
Since you're idling when you performed this test, the available vacuum won't replenish quickly.
Try this: pump on the brakes when the car is OFF. Then with your foot firmly pressing on the brake pedal, turn the engine on. The brake pedal should lower due to available vacuum or booster assist, and you should feel the "normal" brake pedal feel.