Time for an update.
I had been looking for a shop, that could machine the pump body for an oversized Sonnax pressure regulated valve in Poland. With no success. Therefore I've ordered a 3/8 reamer in US on Ebay (funny tho it came with a "MADE IN POLAND" mark), and a 0.375 drill to fabricate the tool for machining the pump body. Once I got these, I've dropped them to a nearby machinists. I help them with CNC electronics, they help me with machining stuff. It took quite a time, but eventually they made the polished liner, that snugly fits the pump bore and the reamer:

Further, the liner has been machined inside at one end, to provide sort of a pocket for the reamer:

This is necessary, otherwise the reamer shaft would be too short to pass through the pump body.

 

For those who are devoted to DIY here is a setup procedure.
1. Fit the tool into the pump bore.
2. Mount the pump body into the vice, do not tighten much.
3. Using a nylon hammer compensate the left-right tilt:

We managed to set the tilt at 0.01mm
4. Check the front-back tilt:


In our case it was just perfect (within 0.01mm), so there was no need of correction.
5. Tightened the vice and check the tilts over again.
6. Center the tool over the bore, by jogging the CNC X-Y:

Again, we were in 0,.01mm.
7. Confirm with the bore. Surprisingly, we found the bore to be... oval

8. You may install the reamer. IMPORTANT NOTICE: If You have been changing the vice tightening while doing these steps, You have to re-check all over again. Otherwise, You're ready to rock and roll.

9. Set the spindle RPM for 150 and start machining. Keep it well lubricated.

 

 

And time for the results. Bear in mind to seal the orifice inside the valve:



Sonnax says to use a pencil rubber, but I've used a sticker instead. My measuring plate would cover the orifice location from top and I wouldn't have a good access there.

-0.65bar which is -19.19Hg. So, machining was successful (otherwise I wouldn't publish the above method).

 

Bravo Mike!!!