After the car sits overnight, the brake pedal is very hard to press. It lacks pressure for the first ten seconds, and then it is normal. I would say this is the check valve for the brake booster. But this is a 2006 Volvo XC90, and I have read that the thing that looks like a check valve is not actually a check valve–it’s simply an elbow that connects the booster to a hose. What (and how) do I replace whatever is acting like a check valve?
Start the engine, allow the brake booster to pump down. Remove the elbow… is there a giant sucking sound right before the engine dies? Should be. Now quickly run around and check the brakes. Power? Then the check valve is INside the booster and you must replace the booster. No power? then the check valve may be where the hose attaches to the manifold. I’d consider that a bad design but stranger things have happened.
Rockauto shows no check valve being separately available so I think it is in the booster.
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