Why isn't gulp filter filtering fewer files in my bower packages

I am trying to get all files in my bower components using the main bower files and filtering them by type using gulp-filter . it works great when i filter js files like this:

gulp.task('bower_js', function() {
  var jsFilter = gulpFilter('**/*.js');
  gulp.src(mainBowerFiles(), {base: './bower_components'})
    .pipe(jsFilter)
    .pipe(concat('vendor.js'))
    .pipe(gulp.dest('./_public/js'));
});

      

        
        
        
      

    

however doing a similar task for a smaller amount doesn't work (i.e. nothing goes through the filter):

gulp.task('bower_css', function() {
  var lessFilter = gulpFilter('**/*.less');
  gulp.src(mainBowerFiles(), {base: './bower_components'})    
    .pipe(lessFilter)
     // .pipe(less()) commented to narrow down the problem
    .pipe(concat('app.css'))
    .pipe(gulp.dest('./_public/css'));
});

      

        
        
        
      

    

although if I run mainBowerFiles without filter, it dumps the contents of the .less

files to the destination folder.

to give a concrete example .. this is the relevant part of bower.json

one of my bower: bootstrap packages:

  "main": [
    "less/bootstrap.less",
    "dist/css/bootstrap.css",
    "dist/js/bootstrap.js",
    "dist/fonts/glyphicons-halflings-regular.eot",
    "dist/fonts/glyphicons-halflings-regular.svg",
    "dist/fonts/glyphicons-halflings-regular.ttf",
    "dist/fonts/glyphicons-halflings-regular.woff"
  ],

      

        
        
        
      

    

bootstrap.less

in turn imports other files:

// Core variables and mixins
@import "variables.less";
@import "mixins.less";

// Reset and dependencies
@import "normalize.less";
@import "print.less";
..

      

        
        
        
      

    

etc.

any ideas?