A method for leak-testing a pipeline system, of the type having a test tee fitting in a standpipe, includes attachment of a leak-testing device to an opening of the test tee. A water hose is coupled to a threaded bushing of the leak-testing device for passing water through a one-way valve into the interconnected pipeline system. After leak testing is completed, a sealing cap is secured over the threaded bushing, and the device is left permanently in place after the test is completed.