the lines are retrace lines. if you had weak caps before, and all your adjustment pots were cranked to compensate, your levels will be way off after capping.
my rule about brightness is to turn it down just enough to where black backgrounds do not show up in the CRT, black should be virtually invisible -- you should see no "black dots" in the raster.
contrast, likewise, you don't ever want that turned up super high, cause it increases the chances of burn-in (high brightness will do the same), put unnecessary stress on the tube, and will be a smeared bundle of shit anyway. get a screen that just has like red, green, yellow or white text, gradually turn your contrast up to where you start seeing bleeding off the side of the text. then keep turning it down until the bleeding disappears.
for both of these functions you have some "master" controls available as well: the Screen pot on the flyback being the main brightness setting. if you're ever unable to get a picture to be bright enough or you can't get it dark enough, you have to adjust the Screen pot accordingly to find that happy medium to get your Brightness pot where you can adjust it within acceptable spec.
after a cap kit, I don't think it's so likely much on a newer monitor like this one, but if you notice your white balance is off at all, you'll have to flip some of the necessary bias pots for the R G B on the neckboard.
I'd work on turning the Screen down first. that will create retrace lines with great ease. find a black background like a test screen or something to adjust it with.