While I'm driving the van, and want to slow down, the brakes are very hard to press. It occurs after only half an hour of driving. I'm not sure what is causing this. I had the ABS system replaced. The brake lines are new as well. I have checked if there are any vacuum leaks but there are none. What else can I check?
If this is not related to any vacuum leaks, the next place to check would be the brake booster itself. As you press the brake pedal down, a shaft inside the brake booster moves forward into a diaphragm, which separates air into one side of the booster. This increase in pressure is what allows the braking assistance at the brake master cylinder. If any of the components of the brake booster are not working properly, this function will fail creating a difficult to press brake pedal as you describe. I would recommend having an expert from YourMechanic come to your car's location to diagnose the hard-to-push brake pedal.
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