Most of the time it is just a bad module. But did the module fail because of a bad valve? The shop is playing it safe by replacing both. Plus replacing both is faster and easier. The unit is tucked under the truck and it is faster to remove the brake lines, the mounting bolts and just pull the entire unit, install the replacement and bleed the system.
However, it is not a hard DIY item to change just the module. It is just a tight fit. The advantage of swapping just the module is that 99% of the time you do not even need to bleed the system when you are done.
As for the parts, you can buy a reman unit cheaper, or go visit a you pull it type yard and remove a couple. That would also show you how they come off without disturbing yours.
Oh, instead of removing the battery cable, you can just remove the ABS fuse. It will kill power to the system and stop it. You will not have ABS but you do not with the light on anyway.
Monday, August 14th, 2017 AT 12:39 PM