My 97 f150 xlt has had a recent problem, when I cold start my truck and press the brake while still in park my RPM dips down by about 100-200, and when I’m in gear it will completely stall all together when the brake is pressed for long enough, I have replaced the brake booster already but to no avail, could this be a vacuum leak in the hose itself?

 

If you can, Have someone look at your rear stoplights when you step on the brake and hold it down. Do they see anything unusual regarding the intensity of the lights?

 

Connect a vacuum gauge to a tap off the manifold.Work the brakes and see what the vacuum reading indicates. It should stay steady between 18-22 inches. A quick check after you shut the engine down. After about ten minutes, pump the brakes, you should get atleast three assisted pumps before the pedal goes hard.

 

I know you stated you replaced the brake booster,
Are you sure your truck doesn't have a hydroboost unit?