The front ones are vented because the front lines feed into 2 way check valves on the service line and a regular check and a 2way check on the emergency line. If you have noticeable air leaking out at the front gladhand vents, you need to track down which valve is leaking back and fix it... If those are leaking and the gladhands get capped, air pressure will build up in those lines and it can hold a 2 way check in the wrong position which can interfere with normal brake operation...