Spongy Brakes

[usa-69z]

Can some-one help with this, my pedal has always been very spongy since i moved to a 7/8 master.. my brakes are manual , pedal ratio changed ,, just installed willwood 4 pot 'dynalite discs' and calipers up front and 'right stuff' single piston rear calipers .. Master is wilwood , and using a summit proportioning valve. I did email wilwood and they have suggested that the 7/8 master may not be enuf with the larger rear piston calipers..

could this be correct?? i need more pedal..


Gav