Hi Justin,
I converted my 51 half ton truck over to a dual master cylinder recently. It has worked flawlessly so far.
I have all drum brakes and fabricated all new hard lines. The original lines didn't work in my case.
I seem to remember that when I purchased my dual master cylinder there was one for disc brakes and one for drum brakes. that would be worth checking into.
Also, did you put in new flexible lines in the front? The old rubber lines tend to expand.
The main reason brakes get spngy is that there is air in the lines. If you are bleeding down by pushing the pedal you might not get all the air out. The way to go would be to pressure bleed up from the individual wheel cylinders (or calipers if you have discs). This is a simple and cheap way to go. I made a pressure bleeder from a bug sprayer bottle from Lowes and a length of fuel line. Simply put fluid in the bleeder tank and pump it up a little and attach it to the wheel cylinder. Be sure to remove the cover from the master cylinder and crack open the bleeder screw.
Careful though, if you use too much pressure you'll get a geyser and ruin that nice paint!
Also start from the furtherest wheel and work your way back.
Hope that helps,
Reso-Joe
