Engine is reving up to 6000 RPMs after changing the brake booster?

Hi,

Since you replaced the brake booster, it could be related to an engine vacuum leak. The brake booster receives an engine vacuum to operate. If you left it disconnected or if the vacuum hose is damaged, it can cause idle issues. Also, on the rear of the brake master cylinder is an O-ring which is a seal for the booster. It has to be installed and not leaking.
However, 6K RPMs are extreme.

Take a look through this link to get started. It explains how to find and repair a vacuum leak.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If you look at pic 1, I highlighted where the vacuum hose from the engine to the booster will connect. Pic 2 shows the O-ring seal on the rear of the master cylinder. Check both of these. Also, follow the vacuum hose back to the engine to confirm there are no breaks in it and it is properly connected at the engine.

Let me know if this helps or if you have other questions.

Take care,

Joe

See pics below.