Steps to reproduce

1. create create ant build with parametrized boolean var "SKIP_SONAR"
> the Jobs run executes and shows correct that i checked "SKIP_SONAR" checkbox before run.
[myProject] $ cmd.exe /C '"D:\buildenv\tools\ant\bin\ant.bat -file build.xml -DSKIP_SONAR=true distribute && exit %%ERRORLEVEL%%"'

2. after ant build step insert single conistional buil step RUN? "NOT", !Boolean condition, Token SKIP_SONAR, Builder: Invoke Standalone Sonar Analysis.

The condition evaluatest correct to "[Not] enabling perform for step" but executes the steps anyway. But the NOT shall make i NOT happen.
see log
>BUILD SUCCESSFUL
>Total time: 1 minute 24 seconds
>Run condition [Not] enabling perform for step [Invoke Standalone Sonar >Analysis]
>[workspace] $ D:\srv\sonar-runner-2.3\bin\sonar-runner.bat >Dsonar.jdbc.driver=com.mysql.jdbc.Driver ">Dsonar.jdbc.url=jdbc:mysql://mysqlsrv.loc:3306/sonar?>useUnicode=true&characterEncoding=utf8&rewriteBatchedStatements=true" >******** ******** -Dsonar.host.url=http://mysonarsrv.loc ******** ******** >-Dsonar.projectBaseDir=D:\srv\usrhome\hudson\jobs\myproject\workspace ->Dsonar.sourceEncoding=ISO-8859-1 -Dsonar.sources=src -Dsonar.language=java >-Dsonar.binaries=build/classes -Dsonar.projectVersion=1.0.0 ->sonar.scm.url=scm:svn:https://mysv.loc/svn/myproject/trunk ->Dsonar.tests=unittest -Dsonar.projectName=myproject
>AD:\srv\sonar-runner-2.3
>ASonarQube Runner 2.3
>AJava 1.7.0_25 Oracle Corporation (32-bit)
>A[Not] enabling perform for stepWindows Server 2008 R2 6.1 x86
>INFO: Runner configuration file: D:\srv\sonar-runner-2.3\conf\sonar->runner.properties
>INFO: Project configuration file: NONE
>INFO: Default locale: "en_US", source code encoding: "ISO-8859-1"
>INFO: Work directory: >D:\srv\usrhome\hudson\jobs\ArchitectureModules_TRUNK\workspace\.sonar
>INFO: SonarQube Server 3.5.1
>09:41:09.704 INFO - Load batch settings
>09:41:10.801 INFO - User cache: C