Brake booster

[mcramsay]

I have the Marlin supplied dual brake booster, coupled up to a Sierra master cylinder. With the engine off pedal goes rock hard with only a little bit of force on the pedal. However with the engine on the booster is doing its job so well the pedal goes to around 3/4 travel before you actually get any feed back. It's almost like the booster is working too well

I know this is a widely covered topic on her so how do others find it? Have I got something wrong on my set up? It's hard to tell how much braking I'm doing as the car is only creeping forward and back on the driveway!