I am going to remove the VB and start with a front clutch air check at 150 and 40 psi to simulate R and 3rd under the assumption that if 1&2 hold so well I must have some line pressure in D and more in R. If it’s the piston seal I’m thinking I might hear engagement at high psi but disengagement will be almost instant instead of a quick sigh and at low pressure I’ll get no thud just air rushing.

If it seems to hold at both psi and not leak more than it should then I will dig in to the valve body and look for a backward piston or missing check ball. I stayed up late studying my trans books- I think I sorta have a handle on the trans circuit operations. I need to study up on valve body operation. I think the selector& throttle pressure are working and the governor vs line pressure is properly controlling the upshifts in D, at least until it needs to go to 3rd.

I never had a high stall converter before or built an auto before so I am not sure if it is sort of selecting D then slipping or what, but a lack of front clutch apply would seem to be a good candidate for my troubles. I remember doing the whole electrical tape and 15 mins in the freezer thing on the piston to try and get the seal to face down on the way in and I lubed it but did I air check it I dunno. I don’t remember it being particularly easy to install but I didn’t think I forced it…

In any case this is a new build with a handmade floor, firewall, console, etc so I was tempted to finish sealing and painting the bottom on a buddys lift but now I’m motivated to do it right- call this a successful mock up, take the body off to nicely detail and protect the underside, and have access from above to easily R&R the trans.

Thanks fellas for helping keep me moving forward in the right direction! I’ve been building this truck since May and despite some setbacks it’s really starting to come together now. The 512 is strong and the breakin oil and filter looked good, it steers, stops, and goes like a bat outta hell. Always more work to do.