why does my 04 dodge ram breaks lock up?

no, but i scotched front wheels, removed the rear wheels and with truck running and in gear about 30mph,i hit break peddle.and held down hard. the rear breaks did not engage for 4 seconds and only stayed for a second .the hole time the front calipers were engaged tight. do you think it is the anti lock or proportion valve ?

I would think master cylinder,or bad brake line,s.

Could

Be eather. Did your Absolute light come on?

Jl. I think im going to replace break lines. It doesn't have proportion valve,it has the proportion module. Which cost$300 from dodge.