1/4 " and 3/8" barbed inline ball check valves printed in one piece with an air-soft bb (any 6mm ball ) as the check ball . I'm going to share an example of my g code and a rough outline here of how I did this .

so the basic setup for this is to print the file so that the arrow points upwards on the z axis .

slice the file for your printer

save the g code to file ( can be on the sd card )

open the g code file with notepad or equivalent ( usually want a bare bones text editor )

this part differs depending on size

1/4 barb is layer 138 (21.4mm at 0.15 layer height )
3/8 barb is layer 130 (18.5mm)

use the " find" function to locate the required lines . ( search for" layer :131" for example)

now at this point the code is specific to where the part is placed on the print bed and other factors .

you will need to an an "M0" just after the end of the previous layer so its the first thing that happens after the Z movement . ( witch ads a wait for button press into the program )

and I also ad a line similar to this as well

G1 F858 X60 ( this moves the print head out of the way,60 is just an example that worked )
M0 ( this makes the machine stop and wait for you to press the encoder button )

you could modify the Z axis as well if you like

G1 F858 X60 Z 22.5 ( moves head up 1mm and to the 60mm point on the x axis )
M0 ( wait for button press )
G1 Z21.4 ( moves z axis back to original height )

once the file is saved it can be loaded to an sd card and printed like normal, when the desired line occurs the printer will move out of the way

drop the bb in the hole , and click the encoder button

print should continue and seal the ball inside

finished part the ball should be loose and spin and rattle when blown foreword and should restrict flow in the reverse direction.

I plan on explaining this further in a video for those who don't already understand gcode .