I found the info over at mercedesshop a while back and built one of the bottle jack style. Drilling and tapping (3/4" npt) the cap could be a problem for some, but I had a friend who owns an industrial machine shop do it for me.
If I build another one it will be the grease gun style. It looks like it should be a lot easier. I would also spend a little extra for a better gauge with a needle that shows what your maximum pressure was during a test.
c&p:
"Credit goes out to OLD300D who sent me this a while ago.
Pop tester parts list
--------------------------------------------------------------------------------
I bought all the parts from
www.McMaster.com
1 8803T51 Bottle Jack ($12.74)
1 4000K763 3000psi gauge, 3/4" NPT ($10.04)
1 5269K311 Metric fitting, 1/8" NPT to 6mm tube ($3.12)
1 50925K197 Steel T fitting, 1/4" NPT, MxFxF ($4.27)
1 4513K341 Hex bushing, 1/4" NPT to 1/8" NPT ($0.85)
1 4513K347 Hex bushing, 3/4" NPT to 1/2" NPT ($1.18)
1 4513K344 Hex bushing, 1/2" NPT to 1/4" NPT ($0.95)
The jack must be disassembled, and the pressure relief in the cylinder plugged. I tapped it, JB welded a set screw into it, then ground it flat. Remove the seal from the top nut and tap it 3/4" NPT. I JB welded the bushing into this thread as well. The gauge and 1/8" NPT bushing thread into the T, the injector line is cut and is fitted into the metric tube adapter. I also tapped and fit the bottle jack for a funnel into the reservoir to keep fluid in the cylinder, as quite a bit is pumped out as you test injectors. I used ATF and Marvel mystery oil to fill the tester."
The 5269K311 Metric fitting, 1/8" NPT to 6mm tube fitting is for mercedes injectors. It is probably a different size for a ford.
David
edit: I think that the thread size on a grease gun is 1/8" npt.