Brake Question

[nickz84]

I have a 79 gmc heavy half and I am told the brake booster is bad. I have no way of telling if it is but when i stop the brakes go to the floor and the red light comes on where the gas gauge is. After I do stop they just keep going til they hit the floor. Now on to the next question I want to buy a new booster just to be safe but there is 2 different kinds for that year jb3 and jb5 the database that autozone has says to look in glove comp. or under hood and it's not in either place. SO any help with this matter would be appreciated thanks in advance.