if i need a better pilot, i can put the blowgun back onto a 3/4" modded sprinkler valve
I have found that unless the piston is really tight (and i mean REALLY tight) you will probably need a better pilot. I know blowgun valves work for a lot of people, but i have never had good luck with mine. (maybe I should switch brands

.)
This looks really great so far. It is extremely clean which gives huge bonus points in my book. One thing i am not liking so much, is the inlet tapped into the elbow. This may prevent you from going to 150 psi. I wouldn't trust a self tap in PVC anywhere above 100 psi, especially not in an elbow at that angle.
Secondly, double check the clean out cap. I find that most clean outs are not pvc. There is also another self tap there that i wouldn't trust. You should think about threaded bushings. I know they will give you a little more pilot volume, but they are definitely worth it safety wise. If you do find that the clean out is DWV not PVC, then you will need to buy something else anyway.
Overall, for someone as new here as you, (you have probably been lurking for a while anyway) this is a very clean, well thought out and ambitious project. I cannot wait for it to be finished, as i am planning a large bore blowforward breach semi myself.
Great Job
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