I HAVE A 5.0 MUSTANG AND THE RPM DROP WHEN I STEP ON BRAKE.

 

I am guessing you have maybe a 92? What mods do you have? What have you done recently? Did it just start doing this, or has it gradually gotten worse? We need a little more info please.

 

I HAVE A 92 LX , FEW MODS LIKE UNDER DRIVE PULLEYS, AIR INTAKE, 70 MMTB, IT HAS BEEN DOING THIS FOR AWHILE . SOMEONE TOLD ME IT WAS THE BRK BOOSTER, IS THIS POSSIBLE

 

Theres a Vacuum Hose that goes from the brake booster to the intake manifold... check to see if its loose, or possible has a hole in it... my car was doin that as well, and it turns out that vacuum hose had a hole in it

 

I HAVE REPLACED IT THAT HOSE ALONG WITH SOME OTHERS.

 

Does the RPM's drop a small amout or is there a major drop? You shouldbe able to see a small drop in RPM's but nothing major in a normal system. If it is major check the brake booster for leaks.