I am delighted to take the correction with regards to vacuum. In fact, it makes my life a little easier for a different reason. However, my advice still stands, I think. Fixing something that is not the problem makes no sense when there are viable ways to find out what the problem really is. I maintain that problems on two cylinders only, numbers four and six, are unlikely to be caused by the booster. I have the same unit. My engine ran fine without any brakes when the booster unit failed because the vacuum tank was fine. The booster unit would not retract when the brakes were applied but vacuum in the tank was constant otherwise. If the tank is at fault, or the hoses, then it is not the booster unit. It could be a valve within the booster unit. Or, it's something else. If the engine cannot create sufficient vacuum in the tank, fixing the booster unit will take time, money, effort and disruption and achieve nothing. Hugh