When you press the brake, you use the engine vacuum to assist braking via the vacuum brake booster behind the brake master cylinder. Since this is when you notice the issue, it sounds like you may have a vacuum leak at the booster or elsewhere under the hood. A large loss of engine vacuum will cause the engine to run poorly, rough, and even stall.
Let's start by testing for a leak. Here is a link that explains how to locate and repair an engine vacuum leak. Take a look through it and let me know if you are comfortable performing the procedure.
Let me know what you find.
Tuesday, February 14th, 2023 AT 5:03 PM