After looking at the image closer, the 2 mounting bolts at an angle are a dead give away. It is a spool valve used for a tractor protection valve. If you don't have a trailer, you don't need it anyway, but they are still available.
http://www.peterbiltparts.com/brake...706bxw/?gclid=CJCzmsON7rwCFS1p7AodagsA_QIf it runs, I wouldn't worry about compression readings. Anything with air brakes is handy to have because air brakes are so much simpler to maintain than hydraulic. And you have to remember that even if you have air leaks, you have a compressor to replenish the air. When hydraulic fluid is gone, its gone and you have no brakes.
