Sounds like you should spend some time figuring out why they won't hold. I can set my e-brake then put it in gear and try to throttle forward and the truck won't budge. It has alot to do with the distance the cable housing bracket is from the lever arm. You can then fine tune the adjustment with the cable tension. In my case it probably helped that I personally rebuilt the calipers so I KNOW they work correctly.
I do need to look at this. I bet my issue is how my rear disc cable bracket is. they are not welded, or anything...just sitting there under the axle.. so maybe it not held in place and cable pulling...
flame on...