Jim .... I have successfully installed two split boots and they did not leak. The first key is to align the split to the rear at the 9:00 o'clock position. That is the place where it will flex the least. Second, I clean and glue the split with weatherstrip adhesive before clamping it together with the hog nose clamps.
You didn't mention how your replace the boot; I remove the gas tank to gain good access. I suspect that the boot is leaking at the split; you could remove the clamps, clean and glue the split and reclamp your current boot. That should fix it.
Jim Villers
190SL, 230SL 5-Speed, 190E 2.3-16 Kompressor, MGB 5-Speed