To close the shutter again after a long exposure you need to half (actually less than half, maybe 1/4) cock the mirror cocking lever, if you listen carefully you will hear it closing.
P.S. I've found the Canon dual cable shutter release works perfectly on my RB67 Pro SD...and I paid it about half price compared to the Mamiya one.
I recommend putting the lens back on prior to cocking the lever to avoid any shake. As for cable releases, get the cheapest one and use the "mirror up" option on the lens. Way cheaper than the dual release.