73-87 Rear Disc Brake Conversion Problem

[SmokinB]

might not be the right volumes being used
Maybe a brake line diameter or bad piston seal(s) situation
The front calipers are still the original non-low drag brakes right? Hows your pushrod lengths

I assume you have rubber whips from the steel lines to the calipers. These can cause sponginess. If your not using a very stiff steel braided whips you may want to switch them.

I've had this same problem and this was my fix.

I've never paid attention to brake line size but intuition tells me that if they are not the same size the larger lines should feed the more important front brakes.

 

[Blazerbiker]

I'd second the 1 1/8" master. 1" will give you a pretty long pedal stroke.