Techniques to identify air leaks in brake system? (spongy after sitting)

This pertains to a motorcycle, but same general concept. Had a recurring issue with an SV650 w/GSXR1000 front end where the front lever would get spongy after only a few days to a week of sitting. I replaced the lines with stainless steel, new copper banjo crush washers, re-bled, still did it. Swapped the bleeders, tried again, still did it. Can't for the life of me remember whether it did this before the master cylinder swap I did (weird that I can't), so I can't eliminate the master cylinder.

Unfortunately, it holds pressure overnight with a zip-tied front lever. No visible brake fluid leaks. Stops just fine when it's bled. I need some ideas to identify whether I have a messed up bleeder seat, bad master cylinder seals, maybe bad piston seals, etc. I just re-bled, gonna let it sit overnight then try to bleed tomorrow and identify which bleed location makes the lever hard again.

Otherwise, I might have to find a way to cap off the lines to test the master cylinder. Any other ideas?

I'd suspect parts highest in elevation. Even at rest, the brake caliper has a column of fluid sitting on top of it (ie greater than atmospheric pressure), and it seems backwards to me for air at atmospheric to push into a system at a higher pressure.

The master in the other hand, will not see that same problem.

Robbie wrote: I'd suspect parts highest in elevation. Even at rest, the brake caliper has a column of fluid sitting on top of it (ie greater than atmospheric pressure), and it seems backwards to me for air at atmospheric to push into a system at a higher pressure. The master in the other hand, will not see that same problem.

Great point. I will try to find a master to swap in.

If nothing is leaking, is it possible the pistons are making their way back into the calipers? If so it would work fine after a few pumps of the lever to get the pads out near the rotors again.

I have seen a few wheel cylinders that never leaked a drop but would suck air over time. I suppose it's possible that a master cylinder or caliper could do the same thing.