Brake pedal goes lower when I am not accelerating

There are a number of things that can cause this but based on what you have done the brake booster is what makes the most sense.

However, here is a guide that will help with this:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

I am attaching the testing for this as well from the manual and the process to replace it as I suspect this is the issue.

Please run through this info and let me know if you have questions.

Thanks

The only connection that the brakes and engine have is the brake booster as it uses engine vacuum.

So, I suspect you have an internal leak in the booster. I understand it may have passed the functional test but that is the only thing that makes sense based on what you are telling me.

When you are not accelerating that is when the engine vacuum is at the highest. So, if the pedal is dropping more than it should before it applies the brakes makes me think we still have an issue in the booster.

However, let's take a step back and find out how this started. Did you just buy this vehicle, and you feel it is too low? Or did you do a repair and then this started?

Just let me know the history of this and we can go from there.

Thanks

Okay. So, when did this issue start? When you put the calipers on? If that is the case, we could have a caliper issue.

Use vice grips and crimp off the rubber brake line one at a time and then retest the pedal. When it feels normal again, you eliminated the problem.