Well, it is the Rennbay kit - I "mis-remembered" when I mentioned it in my original post.


Rennbay ps rebuild kit

 

 

Had same kit and same issue... that's how I knew. Either it is something with the way I installed it, the kit compatibility with the pump that I had, or something I did with the pump that perpetuated the situation. I ended up shelving the pump and sourcing a used one out of frustration.

New reservoir, wheels in air, bled system -- worked for a bit and then didn't, same symptoms, same issue.

 

The photos in post #14 clearly shows the problem.
The plastic (dumbell shaped) gasket reinforcement is supposed to fit into a groove in the rubber (dumbell shaped) gasket.
The post #14 photos show the plastic reinforcement outside of the gasket, which is allowing the unsupported rubber gasket to get sucked in.

 

The plastic dumbell-shaped piece does fit on a lip molded into in the rubber gasket (and probably wasn't seated fully when I took the reassembled-picture above). However, there is no way for the plastic piece to be inside the rubber gasket (as in closer to the center of the pump) nor within the rubber gasket (as in surrounded on all sides by rubber gasket material). The plastic piece seems to be perfectly engineered to follow the outer perimeter of the channel inside which the rubber gasket sits.

The YouTube video above is the exact video I used during the intial rebuild.

I still had the old gasket handy on my workbench, so I snapped some new pics. I cranked up the contrast so the lip in the rubber gasket onto which the plastic piece sits can be discerned.


OLD rubber gasket and plastic dumbell-piece.
The "handle" of the dumbell is irregular in shape. Either part could be rotated 180 degrees while remaining flat and still allow the deliberate irregularities to fit together.
However, if one of the parts is flipped over, they will no longer engage.
The channel in the pump has corresponding deliberate irregularities.




Knife blade points to lip along entire outer perimeter of rubber gasket onto which plastic piece sits.
This lip exists along a single face (the "bottom" as pictured here) of the rubber gasket.



Plastic piece installed onto rubber gasket. The plastic piece extends all the way to the "top" surface of the rubber gasket,
though that is difficult to see in this photo.


I figure the precise fitment of the old components with each other has something to do with the 31 years they have spent together.