I found a solution. The error in my approach is that I checked out the projectmaster as a working copy on it's own.
Instead I have to use externals. That is I needed to define a svn:externals property on the root folder of my project working copy, that links the subfolder "projectmaster" to the external svn repository of my projectmaster.
As soon as that is done checking out or updating the project automatically checks out or updates the external.
That way teamcity will be able to build my project.
Sascha