Help! Bubbles in master cylinder (1 reservoir only)

[mbeers6]

Hello all,
I've been struggling with a spongy brake peddle in my cobra and can't find the cause. This is a non power assisted brake setup, dual reservoir, Frenco master Cyliner (autozone) B&B kit car wth about 3k miles.

My first master cylinder held pressure fine, good firm peddle, but leaked at the lid. I replaced it with a new (not remanufactured) Fenco master cyliner.
-Bench bled until no bubbles.
-Installed in vehicle with teflon tape at brake line fittings
-blead brakes using helper and they felt ok.
-test drove and pedal went to the floor.

I tried to rebleed the brakes using vacuum pump, got some bubbles out, but still very mushy pedal.

I noticed with the cap off the master cyliner that as soon as I push the pedal down the small reservoir (rear brakes) burps up fluid in a small stream. The larger reservoir has no bubbles. Does this mean the air leak is confined to the rear brakes?

I know there is way to narrow down where the leak is coming from by putting a clamp on the rubber hose at each caliper. Can someone explain the steps?

My next thought is to take out the speed bleeders currently in the calipers, replaced with normal bleeders with teflon tape. Then maybe gravity blead the system.

Any other ideas? Bleeders are at top of the caliper, and I tried tapping the calipers to release air bubbles.