do I need a larger MS to run 4 wheel disc. Right now I have the Alumunin 15/16 one and the peddle feels spongy. All 4 calipers are GM metric. This is a round track car that use to have disc front and drum rears.

Try a 1" master, if pedal feels better but not enough, you have your answer. Tim

how big are the piston(s) in the calipers?

discs don't NEED bigger bores to work...again, it's all about piston ratio to create line pressure.

however, everything I've read says that discs require a longer stroke for fluid volume.

I don't quite understand this myself, since the pistons and pads sit up against the rotor and never retract, so you really only need to create line pressure, not move parts, with discs, but on drums they have return springs which pull the shoes back away from the drum, so when you hit the brakes, you DO have to move parts before you start making pressure and slowing down.

Yes it has the stock distrubation block, It was fine before I put the disc n the back. The pistons are 2.375 which is stock GM metric

The rear piston area should be 1/2 the area of the front pistons.

Got it fixed. Bought a new MC for 1980 dodge its 1.030. Alumun with two hole mount. This is a dirt track car so I dont need much rear. Too much rear brake makes it loose going in. Peddle feels good now.I will find out (tomarrow) Sunday at practice if its ok. Thanks Mike

To answer the question, yes, going with much bigger disc brakes on the rear with a disc/drum master, can give you a spongy pedal.

On my Dakota I went from stock disc drum to 4-piston brembos from a viper on all 4 corners. I left the stock disc/drum master the same.

I had a spongy pedal

Upgraded from the stock 1" bore disc/dr master of the 98 Dakota to the 1 1/8" disc/disc master of an '03/04 Dakota which was a bolt in swap. Perfect pedal feel.

I left the stock "proportioning block" with the "failed brakes" shuttle valve, and the stock RWAL system in place. Under hard braking the RWAL keeps the tail end happy and me out of trouble, since the 4-piston brembos on 13" rotors are more than enough to lock up the light rear end

I know guys who have done just the viper brakes to the front, leaving everything else in place, including the drum rears, and had no problem.

So in my case, yes, going to larger disc brakes on the rear created a spongy pedal.

Thanks for the info! I've got vipers for all 4 (guessing the same as you) - 44/40 and 42/38. Also have a 15/16 manual master. I'll know soon enough if it's wrong I suppose.