I apologize for the late response, as I saw this post a while back and meant to reply... I have seen this before in various caliper designs, especially the late model designs with square cut caliper piston seals. These are indeed actually designed to grip the caliper piston enough to purposely retract the pistons after brake apply to increase fuel economy. Obviously they are not designed to retract the pistons as much as you are describing. Your case seems to be a scenario where the pistons are too dry on the outside of the seals, causing the seals to not allow the pistons to slide as freely as they should. The easiest solution would be to remove your wheels and then force the caliper pistons back into the calipers as far as they will go. Be careful to only do one wheel at a time, as you will likely overflow the MC reservoirs if you do all at one time. Pushing the pistons back in as far as they will go will get the pistons wet with brake fluid in the areas that are currently too dry. Once you pump the pedal back up, the pistons will push back out to where they previously were against the brake pads. Doing this procedure will ever so lightly lubricate the pistons and the seals just enough to prevent them from over retracting as much as you are describing. This should take care of it.
I did have one stubborn case (never fails), which was so crabby that I had to very carefully extend the pistons outward (with the calipers removed from the rotor) with a helper listening very carefully to my instruction as to how much to VERY SLOWLY AND CAREFULLY apply the brakes - emphasis on not wanting to blow a caliper piston out of its bore. I estimated how much I dared to extend the pistons outward (brake pads removed), using an appropriately sized piece of wood to act as an estimated piston stop (this will vary from caliper to caliper so be careful). I managed to get the pistons out pretty far without actually popping them out of their bores, then used a pick to pull back the dust boots and then inserted the tip of a silicone spray can straw in there and shot in the smallest squirt I could muster. I repeated that procedure with every caliper piston to be sure, front and rear, and then was greeted with perfect results after re-assembly. The results were spot on, as the caliper pistons slid right back in almost by hand! I was initially a little concerned about the propellant of the silicone spray possibly acting upon the dust boots to cause swelling, but it did not affect them one bit, even after rechecking a week later (had the customer stop back in for a double check to be sure). He reported back that his condition, same as yours, was completely cured, brakes being rock solid at all times no matter how long or short the car sat.
If you have already done something to correct your problem, let us know what you did and how it may have worked out for you. If you haven't solved the problem yet, give my methods a try and report back as to how it goes. There should be no reason to have to plumb in residual pressure valves...
