As the guy who pioneered the idea of the trailer hub flush system and designed and developed the Hub Flush Kit. www.hubflush.com
I would like to make a few things clear.
None of the trailer manufactures used to offer these kits until we approached them with our Hub Flush kit to re-sell. Most have taken the kit and just copied the idea.
As "Downhilldai" points out, you to can go down to the local pluming store and copy the idea, just like all the trailer companies have done.
The problem with doing so is this. The most important and expensive part of the Hub Flush kit is the nozzle jet which we have designed and developed over many months of testing and redesign. This component which is hand made can not be bought elsewhere.
If you just attach plumbing parts to the back of the hubs which pour water into the hub out of a simple opening, as most of the copied kits do, it just makes it's way to the base of the hub and pours out the bottom. Thorough testing has proved this does little good what-so-ever.
When you consider the money you will save on replacement brake parts and trailer servicing, the proper Hub Flush kit is well worth it.