Steve, Your explanation of the problem is very comprehensive. I see only one detail that you haven't mentioned, so maybe you forgot to do it. You must bench bleed the master cylinder before you install it on the car. Bench bleed kits are available at most auto parts stores and are cheap & disposable. I'm surprised that the vendors don't include them. It basically consists of clear vinyl tubes that loop the outputs back to the reservoir. I held my master cylinder in a large bench vice and pumped the cylinder using a large phillips screwdriver until all the air was cleared out. It is not unlike weight training -- especially if you let the reservoir run dry while you are bleeding it in the car (twice!) like I did. Then you have to start all over again. It is really a two person job. If you are using an opaque rubber hose, try using a piece of clear vinyl tubing so that you can see what is coming out of the bleed screw. There is also a possibility that both master cylinders are defective. The same guy could have made the same mistake on a whole batch. I've had this happen with starters on a GM car. By the time I discovered that my 3rd replacement starter was defective, I was seriously wondering what I could have been overlooking. Let us know what you find. Walt Tampa, FL