I need some help, I have a k machine with about 12,000 hours that takes for ever to pump down to 240 and usually fails because of it. I have changed out the UF pump because it was questionable, replaced the oring on the deair restrictor and put all new hansons on the diasafe. I do see some micro bubbles but nothing that looks like it would prevent it from pulling a negative pressure. I get no flow errors, there are no holes in the bc membranes and it passes the online pressure holding test.

Any ideas where to look would be great.

Is your deaeration pressure OK?
You didn't mention the dialyzer connectors.
Did you try the manually induced positive pressure test to check for leaks?

I usually don't see any bubbles except those leading to the deaeration pump.

Manual Positive Pressure Test......Those bubbles are coming from somewhere. Let it sit for a few mins and start looking for the leak...

If this is an older K machine, where Valve 24/26 are on the same block, check for air there, that's where I have noticed problems. Good luck.
If that don't work definently do the Manual positive pressure test like dave and old F Guy mentioned.

I hate Fresenius machines. I came in this morning, fired it up and no more bubbles. I did double check the deair pressure and it was -24. I guess I will just button it up and hope the problem fixed itself. Thanks for all your responses.

Biomed Dude, I feel for you man. One thing everyone needs to rememeber to not overlook is your concentrate lines/jugs connectors. a couple weeks back I was having a h*ll of a time figuring out what was causing this. It turned out to be a bad bicarb jug cap. The spike/cap would seal good enough to pull bicarb to get into condo but if you looked close there were small air bubbles (BB sized)pulling thru with the concentrate. This was just enough air to raise h*ll with the neg pressure test. Now this is one of the first thigns I look for when this problem shows up. So word to the wise, when you get this type of alarm, try different concentrate jugs before you tear your machine apart.