My '57 Dodge has upgraded manual brakes, later rear with drum brakes and front disc conversion using a factory tandem MC. After the conversion I bench bled the new MC and thoroughly bled the system and had a good firm pedal. I've been driving it occasionally with no brake problems.

So, the other day I jump in and back it out of the garage, apply the brakes and the pedal is way low and spongy. I check the fluid level and look for signs of a leak; fluid level is correct and no leaks.

I have a friend operate the brake pedal while I observe the fluid in the reservoir, rear reservoir shows a normal slight burble in the fluid surface as the pedal is applied but the front reservoir (for the rear drum brakes) has air bubbles rising. Slowly pumping the pedal produces rising air bubbles with each stroke. So, today I sat in the car and slowly pumped the brake pedal numerous times and now the pedal feel is just like it was before the incident.

I'm at a loss to figure where the air in the front reservoir came from, the MC has no RPV so I installed one (10 psi) in the line to the rear brakes. How can air suddenly appear in a system that's sealed tight? Any thoughts?

what did the level do? when you were done pumping the air out, was it low.
I would expect it to be, but just want to be sure.

and it is unlikely that it migrated very far, but I would still be checking rubber lines.

Fluid level remains the same, all rubber hoses are brand new and dry.

odd. logically that air had to displace some fluid.
maybe it was just too little to tell.

I am at a loss.
the only thought would be perhaps air bubbles worked their way up. like microscopic ones after sitting a long time.
but even then I would expect some sponginess.
unless it was like carbonation where it was absorbed into the fluid. but I didn't think brake fluid could do that.

while it is possible to have a rubber hose or perhaps a bad seal act as a check valve only allowing air in but not let fluid out, I can't see why it would stop letting air in too.

I had a single piston ('63) do that to me on a car that is in progress this summer. Left it to work on it at another time and a few days later it was leaking out the back.

Sort of similar. Good luck.

I'm thinking a loose fitting/line/hose that is allowing air to be sucked back up in there on the return stroke.

Yeah either the air is still in there (& I wouldn't think so as there ain't no magic to bleeding) & especially with the amt of the bubbles (from what it sounds like) that it (air) is being drawn back in somehow.

One thing we learned in the Navy was that watertight did not mean airtight.

IOW, air leaks in easier than a liquid. Probably woulcn't hurt to put a wrench on all connections and retighten them.

The air most likely came in from a wheel cylinder.

On my '55 Plymouth, there was an air bubble trapped in the passenger side rear wheel cylinder that would not bleed out. I finally got it out by clamping the flex hose to the rear axle, taking the drum off, and using my pressure bleeder to repeatedly expand the shoes until the wheel cylinder finally burped out the last bit of air. Maybe yours had a stubborn bubble that worked its way up the hard line to come out the MC.

This ^^^^^^

I have had this happen more than once, also VERY common on c3 vett calipers to suck air

The RPV is supposed to prevent that. The notion that the air was always there, trapped somehow, isn't consistent with the pedal being firm after the original installation.

This may sound stupid but had a guy tell me he fixed this problem at the rear wheel cylinders by putting the old wheel cylinder springs in that had the round metal expanders on the end of the springs. I guess some kits dont have the metal rings or people just dont put them in.

Fwiw, I have found some cheap aka Centric wheel cylinders to not have the expanders, they form the end of the spring to take its place, I buy wheel cylinder kits or use the old ones anytime I work on them, check before install.