Got a 76 K5 1 ton with 4 wheel disc. factory booster but with the 1 ton MC. Direct lines to front calipers and Wildwood Adj Prop Valve on the rears. Have SS lines all round. I have great brakes except, it seems, when I flex pretty good. The peddle with get soft and I pump it once or twice and it firms back up nicely. I have power bleed the system twice. What could be causing this problem? I was thinking perhaps the vacuum line outta the booster is being pulled slightly during flex allowing a temporary vacuum leak, then when it's back on relatively flat ground it reseals and builds back up. Any thoughts on this problem would be greatly appreciated.