Did you look at the brake math spreadsheet and find the optimum master cylinder size for those calipers?

I could see a less than perfect size booster/MC causing a problem but from the descrip it sounds like there is a mechanical failure/issue somewhere. A suggestion since the booster (even if undersized) & the MC are good/bled is rehook the MC line to the front discs next to the firewall port only & bleed it out & work with them first & see if I can get it to straighten up

So, I kind of confirmed my suspicions tonight. I eased it over to an empty parking lot right outside my neighborhood, instead of being nice I figured the best vacuum would be achieved by jumping all over it. So, I lit them up from a 20mph roll, backed off and jumped on the brakes...front end dropped and locked both front wheels up, might have locked rear also but I'm unsure. So it's settled, lack of vacuum.

Alright, would that be the eng not generating enough vac (wild cam) or wrong booster?

I'm banking on engine. It's a 318 punched to a 393, hughes HYD roller 236/242 duration with 568/560 lift, 11:1 compression, worked iron 2.02/1.60s, matched air gap topped off with an 850 Holley. So, not real good vacuum.

And it's only going to get worse. I've got a set of fully worked w5s and a matched single plane just waiting to go on, so, my vacuum predicament isn't going to get better.

I ain't into PB but iirc someone (on here) said that either 12 or 14 in hg is needed for good functioning. If that ain't available, is there a booster type that will function with less? Or maybe a vac pump.

Hydro boost, albeit a bit too non period correct looking for me. That is if low vacuum should prove to be the problem. I would myself prefer to go non-power brake in that case.

that'd be my choice (even with adequate vac): less weight/better brake feel/simpler system/more room. EDIT From what I understand PB does not give increased braking, it just requires less leg effort for the input side.

The car isn't factory correct, and I don't mind. If it were a 70-71, I would care, but being a 73 it's subject to my whims. What I want is a solidly streetable car, something you drive like a new car, something that's not laborious to drive. Most of you know what im talking about. I want to Friday night cruise without the pucker factor. I, like many car guys, love my ride, but I don't trust it when traffic gets hairy. I've already started down the pro touring line, now it's time to commit.