tranny brake ....need help

ok this is not a mopar, but my neighbors kids car .the kids only 18 and i have been trying o help him out.. tonight we ran the car, its a monte carlo w/ a sb 406, chevy,,, 4000 converter. msd w/ 2 step... here is the problem.. upon lauch using the t-brake the car jumps nicely. but falls and breaks up. off the foot brake tonight the car ran a new best of 11.29 @117.... 1.59 60ft.... the best 60 ft we got off the t-brake was 1.74....it almost seemed like the two step was comming on late,,any one ever had this problem before ? I did pull the battery out of my ram pickup and put it in the chevy about half way threw the night. his battery did have a bad cell, so voltage was down to a little over 10.5 ...the car did seem to run better then ,but it was into eliminations and we decided to foot brake for the rest of the night ,,,the carb is a 3310 holley w/ vac secondarys.. would a double pumper help or make that bog worse....it got late and we didnt have time to run the trans brake with a fresh battery , so is it possible that it was a low voltage problem?? any help please... i am new to this electronic stuff and am a little lost..

2 nd question ....how do you guys launch with a trans brake... up tight on the converter or set the two step lower and let it flash some? .. gas pedal to the floor when staging ,or just enough to bring to revs up against the converter ? ( at launch rpm ) thanks for the help .....trying to get the kid into the 10s ....