"A few holes" won't get the job done. I would guess you found your cause. You need to make sure the nipple off of the side of the holder is open. A 1/16" drill bit works well for cleaning that out. The hose that connects that to the brass fitting on the manifold can start to clog and create pressure. The brass fitting on the manifold needs to be cleaned as well- again the drill bit works great. Neglect any of those and pressure can build. It doesn't take much to completely unseat one of the seals and then you have a real bad day.

Randy