Cannot bleed 18M974 DSE Master with cap on tight? Anyone else?

[PhillipM]

Here's what I've got going on. AC Delco 18M974 Master and a Tuff Stuff 2229NB Booster. When I started bleeding I had the res full and the cap on tight. Everything went fine for a bit and started to build a pedal then all of the sudden, nothing. Bunch more air in the lines and no amount of bleeding would do anything. I decided to check the master. Take the cap off and put some more fluid in it. Put the cap back on and boom, start getting a pedal again. Then... nothing. I went through this a couple of times before realizing, it was the cap on and off that was making changes. When I leave the cap off or just loosely set on top of the master it bleeds just fine. If I tighten the cap then it's like it starts to get a vacuum and sucks air in from somewhere else. Has anyone been through this? Is the vent in the cap bad? Thoughts?

https://static1.pt-content.com/images/pt/2015/12/IMG_5574_zpslhq7miqb-1.jpg (http://s43.photobucket.com/user/pmckee/media/F85/IMG_5574_zpslhq7miqb.jpg.html)

[a67]

The M/C caps are sealed, not vented. This is due to the hygroscopic nature of most brake fluids. The cap seal(s) have an accordion bellows shape to them to allow the fluid level to drop some from loss of pad material. But with a single cap on a large reservoir only so much can be done.

Bob.