stalls when braking

hello everyone,
i managed to get the car running.
fuel pressure- 60
oil pressure- 60
i am running a mustang hydroboost.
pcm and harness are stock.
idles a bit rough when cold but stabilizes when warm
i drove the car just around the corner and when i hit the brakes RPM drops and dies.
i placed the car in neutral and just step on the brakes and it dies.
start the car and idles fine, then i slowly apply the brakes again and the harder i push on the brakes the faster it dies.
what is/ are causing this?
thanks

Perhaps you wired your brake switch backwards?

Also you may need to raise your idle rpm. When you apply the brakes, you are putting a load on the motor through the hydroboost.

Did you install a vss kit?

yep i have the TPC VSS kit installed.

You should figure out why its idling rough and go from there. Maybe with a solid steady idle it will be fine

well thanks everyone, i raised the idle to 1200 rpm, seem to work. only drops about 50-100 rpm when brakes are applied.
also checked all vacuum ports that i had plugged and installed the vacuum port from throttle body to valve cover vent and it appears to have a better idle.
thanks again

You should not have to raise the base idle to 1200 RPM to prevent stalling when an accessory load is put on your engine. The hydro-boost loads the engine each time you put your foot on the brake. It sounds to me like your idle air control (IAC) is not working correctly or it can't keep up due to the rough running of your engine. You might want to get a scan tool and look at some engine data to see what the TPS is reading and where the IAC is being commanded. With a decent scan tool you can control the IAC to see if it is capable of controlling your idle. The "idle screw" you adjusted is for the minimum air rate and the IAC is supposed to control the idle. I would have to look up the spec for minimum air rate but on a stock engine, it is probably around 400 RPM. Idle on a stock LS1 is about 700 RPM.
Gil

OMG i hope i didnt mess anything up.
i have a scan tool but its really a basic one. i may have to either get HPTUNERS or just have it professionally tuned.
i really don't know anything about tuning..i can replace parts all day...lol
is hptuners really that easy? i have tried watching videos on it but it's not clicking in at all.
thanks

Have you checked for codes with your scan tool?  If you have any codes, this could be your first clue.  Can you read engine data with your scan tool or is it just a code reader?  A good scan tool will be able to read engine data and should have a chart of acceptable values to compare in it.  You haven't messed anything up by turning the base idle screw, that can be easily readjusted.  
    Is the engine stock or has it been modified?  The stock tune in the PCM should run make your car run OK unless you have made significant modifications such as a different cam.  
    Probably the most important thing to make sure of is that all the basics are in order.  Make sure the engine is running on all 8 cylinders, there are no vacuum leaks, all the sensors are reading correctly and their outputs are correct.  Your scan tool should help to confirm this.  
     I agree with you that HP Tuners is very complicated.  HP Tuners is not for the novice unless you are using it to import tunes from someone else.  This is a great feature.   You just need to make sure that the source you import a tune from is trustworthy.    
    Tuning should be the last step to make everything perfect. Be very careful who you select to do your tuning. Make sure they know what your expectations are. Some tuners go for maximum HP on a dynamometer and they ignore the rest of the operating range of the engine. If your car is mostly street driven, you will probably be disappointed. Get the basics right first.  If not, you are adding another variable to the equation that will add to your frustration.  Good luck.  Gil