Ok, heres my suggestion. If the braking system you are using is all from the same truck(frt and rear) Then try to use the master from the same system. If that is not possible find out what the bore sizes are in the oem master and duplicate that. There are a million masters out there that, with an adjustable push rod can be made to work in most cases. Otherwise I would have to agree with Martins statement of a bit of trial and error, sometimes things just work out.
