Did you buy new lines to use on the car for this or did you cut and flare the old line or use feral type fittings for sealing the old lines? If either of the latter two try tightening them up some more and retest
Can you see fluid leaking out anywhere now after using the line loc?
Let us know what you did to fix this and the results
In my installation the lines were all new. There are no leaks, system was properly bleed, and the brakes work perfectly fine in normal driving, pedal holds firm, stops on a dime.
Only issue is when using the line lock.
