How to make the test ball seal better

[waltsyd]

The new 4" test ball is letting water past, very slowly but enough to invalidate the hydrostatic test. I can see every joint but not the test ball (it's in a sealed wye) so I know it's the test ball. I've drained and repositioned plug three times. It's a little better but still leaks slowly. So what do you think of slathering some rectorseal or something else on the plug before inserting? Might harm the rubber? Any other ideas?

[hj]

If you slide it in far enough to be inside the pipe, not a fitting, it will seal when you use enough air pressure.

[Terry]

These are nice

[waltsyd]

Forgot I hadn't posted that the problem was solved. There was a very slow leak at another joint (the only one that touched a bit of dirt hence it took awhile to find it). Job approved and done. Thanks to those who responded!