diff --git a/src/buttons/buttons.js b/src/buttons/buttons.js
index 5848822c82..c7a62d9332 100644
--- a/src/buttons/buttons.js
+++ b/src/buttons/buttons.js
@@ -41,21 +41,25 @@ angular.module('ui.bootstrap.buttons', [])
     require:'ngModel',
     link:function (scope, element, attrs, ngModelCtrl) {
 
-      var trueValue = scope.$eval(attrs.btnCheckboxTrue);
-      var falseValue = scope.$eval(attrs.btnCheckboxFalse);
+      function getTrueValue() {
+        var trueValue = scope.$eval(attrs.btnCheckboxTrue);
+        return angular.isDefined(trueValue) ? trueValue : true;
+      }
 
-      trueValue = angular.isDefined(trueValue) ? trueValue : true;
-      falseValue = angular.isDefined(falseValue) ? falseValue : false;
+      function getFalseValue() {
+        var falseValue = scope.$eval(attrs.btnCheckboxFalse);
+        return angular.isDefined(falseValue) ? falseValue : false;
+      }
 
       //model -> UI
       ngModelCtrl.$render = function () {
-        element.toggleClass(activeClass, angular.equals(ngModelCtrl.$modelValue, trueValue));
+        element.toggleClass(activeClass, angular.equals(ngModelCtrl.$modelValue, getTrueValue()));
       };
 
       //ui->model
       element.bind(toggleEvent, function () {
         scope.$apply(function () {
-          ngModelCtrl.$setViewValue(element.hasClass(activeClass) ? falseValue : trueValue);
+          ngModelCtrl.$setViewValue(element.hasClass(activeClass) ? getFalseValue() : getTrueValue());
           ngModelCtrl.$render();
         });
       });
diff --git a/src/buttons/test/buttons.spec.js b/src/buttons/test/buttons.spec.js
index b7ee29d26c..8fbfa1501e 100644
--- a/src/buttons/test/buttons.spec.js
+++ b/src/buttons/test/buttons.spec.js
@@ -63,6 +63,23 @@ describe('buttons', function () {
       expect($scope.model).toEqual(0);
       expect(btn).not.toHaveClass('active');
     });
+
+    it('should monitor true / false value changes - issue 666', function () {
+
+      $scope.model = 1;
+      $scope.trueVal = 1;
+      var btn = compileButton('<button ng-model="model" btn-checkbox btn-checkbox-true="trueVal">click</button>', $scope);
+
+      expect(btn).toHaveClass('active');
+      expect($scope.model).toEqual(1);
+
+      $scope.model = 2;
+      $scope.trueVal = 2;
+      $scope.$digest();
+
+      expect(btn).toHaveClass('active');
+      expect($scope.model).toEqual(2);
+    });
   });
 
   describe('radio', function () {