I installed a McLeod HTOB a few years back against a big input Toploader. The retailer stated it wouldn't leak. I asked because I was aware of other owner complaints regarding leaks. However, it did leak within 6 months of the installation.
I called McLeod and spoke with the engineer who designed that unit. He told me to send him the unit and when he finished with a modification, he guaranteed me it wouldn't leak. His guarantee sounded like the retailer's promise of no leak. Anyway, during the conversation with the engineer I wanted to know what his fix would be and why he was able to offer the guarantee. He explained they had recently replaced 2 of the 4 internal round O-rings with square O-rings.
So...I sent the HTOB to McLeod, they rebuilt it, and before I installed it, I took it apart to see the mods. And yes sir, there were 2 square O-rings where there used to be round ones. I had kept the round ones before I sent the unit to McLeod for rebuild.
End of story, this rebuilt HTOB has been working great for several years now.