Does anyone know why my Printrbot plus v2 with Rev f board and Lassen firmware does this. The printer homes x and y then goes to the center of the build for z homing. I’ve added G29 to the start G code and there is nothing obvious I’m doing wrong firmware wise. I hope someone can help me figure this out.
That’s actually exactly the way it should be doing it. The sensor might miss the bed or get an inaccurate reading at the outer edges of the print envelope.
My experience with heated beds is that the center is where you want to take your Z measurement. They can bow upwards a couple of thousandths. But on non-heated boards I’m guessing its just to insure that the sensor won’t have any chance of not seeing the platform 
To clarify are you saying it is better to have the sensor go to the center rather than the usual three points? The only reason I wanted all three readings is it seems that the wood under the aluminum bed sometimes bends and warps slightly under the heated bed/tension. If one point is better should I just try to really stay on top of my bed leveling?
It sounds like you have Z_SAFE_HOMING turned on. This is a Marlin feature to insure that it doesn’t try to home Z when the probe is off the bed and is by default set to X-max/2 and Y-max/2.
Every time you perform a G28 Z0, it should go there, but it should still probe the normal three points when you run G29. If it’s not doing that, then something else is going on, but you can turn Z_SAFE_HOMING off in Configuration.h also if you prefer.
A word of warning about the Lassen firmware. I tried it on my GO! v.1 (with LCD) and it set the bed temperature on the ABS preset to 230 C (! - same as the extruder temperature). It sorta seemed like a problem with the EEPROM values getting shifted, but it was not something that seemed to be fixable by setting new EEPROM values or otherwise that I could figure out and j-laird (wasabi_peas on printrbottalk) wasn’t able to figure it out either. Since I was having good luck with the older printrbot_prod firmware I just switched back to it for now. (My board is Rev. D.)
Try
G28 X0 Y0
G29
That is the instructed custom code to place before your slice.
Exactly as everyone else has said – every version of Lassen I’ve published (v1-v3) has behaved this way in response to home all (G28). AFTER it does this, the G29 command should probe three points on the bed, as you expect.
Note also that if you home each axis individually (ex; G28 X0; G28 Y0; G28 Z0;) then the z probe will be performed at the corner of the bed (unless your M212 X and Y values place the probe off the bed at that point, in which case it will complain).
And as John mentioned, he reported problems that I was unable to explain in reviewing the code, nor reproduce as I don’t have a heated bed. There are two other complaints I’ve heard about this build, but they are similiarly restricted to only single users and I’ve been unable to reproduce. I’m still trying to understand/explain these.
If you have any Lassen-specific questions, please feel free to contact me directly – I rarely check this group.
-J