Brake pushrod length.... and other brake woes

If you using a stock style proportioning valve/ combo valve, I have experienced the same issue. I have trained myself to pump the pedal and all is good. I don't have to pump it but it works. This drove me nuts cause I tried changing all brake components. Then I did the same disc brake swap to anther car same configuration, guess what same damn issue with the pedal. This is using all stock stuff, rebuilt and reproduction. I finally got fed up took it down to a mechanic buddy of mine and he bypassed the valve by sending the rear brake linefrom the MC directly to the rear brake line using a brass fitting. ( I have seen this trick before and wondered why people did this, now I know). Much better pedal pressure. My understanding is the prop valve is trying to regulate the brake fluid and a pedal drop on a disc brake car is normal as long as the car will stop. The push rod length should not make a difference. I have used the same push rod when converting from single to dual reservoir and from drum to disc , never noticed a difference. I would say the master cylinder was not bleed properly or is defective. Try to buy new master cylinders, not reman. You can "bench" bleed the master cylinder on the car just watch out for brake fluid spilling. A quick search online will show you how to do it. I have found this to be the easiest way to bleed them.

So in summary
Push rod length is not the problem, especially if you car already was dual circuit.
Try re bleeding the master cylinder
Try a new master cylinder


One more problem I have run into with the disc brake swap, I could not get the Proportioning valve to seal, even using 2 new prop valve from inline tube, Cant remember what I did to solve that problem though, but make sure you are not loosing fluid at the prop valve.