I've been working on cars, trucks and motorcycles for many years but this problem is driving me crazy! I bought an unfinished (But drive-able) project 41' Willys off eBay 1 year ago, it sits on an S-10 chassis. The small-block was new and it had a TH350 that I switched over to a 700 R4. The (Under floor power brakes worked but were low so I installed a 2 lb. residual valve in the front line and a 10 lb. in the rear for the S-10 drum brakes, the brakes worked fine after that. Fast forward 8 months, I decided to install a Ford 9" rear-end that I had (Because I'll install an odd-ball big-block engine in the future). The 9" rear-end has drum brakes so while apart I decided to buy the disc brake conversion from Summit...I also installed a 2 lb. residual valve to replace the 10 lb, and an adjustable proportioning valve. I was back to where I started when I bought the car, low pedal! I thought I had a bad residual valve so I changed it, no difference, I replaced the master cylinder, nothing. The brake system had all 3/16" lines so I replace those lines feeding out of the master cylinder to the front & rear "T's", using 1/4" brake line (Leaving 3/16" going to the F & R wheels). Instead of using an adapter off the master cylinder I replaced the master cylinder with a 1980 Corvette (Larger bore which would bolt directly to the new 1/4" lines), I also re-plumbed the lines where there would be no "High" spots to trap air. Today I will be removing the rear calipers and temporarily positioning then so the bleeder screws will be "Up top", to bleed-out any air, then I'll mount them. If that doesn't work I can only think of finding a place that does "Power bleeding". I've done this kind of work on a few under floor brake systems (Hot-Rods), and NEVER had this many problems! PS: The pedal is higher when the car isn't running, but when it is the power brake diaphragm (Looks like a kit from an aftermarket supplier) is feed engine vacuum, it's low but the brakes do lock (I only tried them while on jackstands). >>>> Any one else had this problem?!