Had a covered hopper on Track 4 yesterday where the brake cylinder pressure crept down about 5 psi over two minutes only during a service application; release looked clean and no audible leak at the ABDX or piping. I soap-tested every joint and swapped the service portion, same result — anyone seen rigging or a hand brake pawl drag cause this kind of slow bleed before I yank the cylinder for a bench test?
Had that exact 5 psi in about 2 minutes on a hopper — drove me nuts — and it was the empty/load valve bleeding the cylinder from a cracked diaphragm; only in service, no soap hit, and swapping the ABDX service portion did nothing. Quick check: cap the BC line at the L/V or bypass it and see if the pressure holds, @OP; if it does, swap the L/V. Caveat: a tired cylinder packing cup can mimic it, but you’ll usually catch a faint hiss at the cylinder vent.
@hill30’s E/L tip is solid; if that checks out, do a quick set-and-chalk — set about 15 psi, chalk the cylinder rod and handbrake wheel, and “watch for piston travel creeping” over two minutes. If the mark walks while pressure decays, the rigging/handbrake is chasing slack (I’ve caught a barely dragging chain guide and a stiff beam equalizer on a hopper), otherwise I’d peek at the emergency portion/accelerator side.