Loosing rear brakes

[garyS-NJ]

On my 1984, i split my uni brake by running two lines to the front and blocking the front line where it goes to the front at the proportioning valve at the rear master cylinder. After that I swapped the front master cylinder with a 1987 master which has a slightly larger bore. The fronts seem a little soft (I attribute to the long rubber line going to the front left caliper and perhaps the marginally sized front master) & the rear was very "wooden" as it was very touchy easily skidding when I touched the pedal. That was fine for a little while but then I lost the rear brake, pedal goes all the way down.... I didn't see a leak so I added some fluid and re-bled it (with the speed bleeder I recently swapped in). I bled out some air and brakes fine for one ride but now the pedal is almost all the way down. Still no leaks but I hear a squishy sound at the rear master cylinder.

I thought a leaky master would do this but only with a visible leak (nothing either under the boot).

That said, can a master cylinder fail like this or is it more likely the proportioning valve (or my Teflon taped plug allowing air in without allowing fluid out?)??. Do most folks splitting the unified brake also remove and block the proportioning valve then stretch the rear brake line to the front brake line spot on the rear master cylinder???

[garyS-NJ]

I swapped rear master cylinder and ditched the proportioning valve connecting the rear line directly where the proportioning valve connected (I plugged the port for the front brake. of course then I bled it and it seems to work fine..