Air in rear brake

[Jims99]

Just rebuilt front calipers on new to me bike.(work great now) Back brakes were spongy when I got it, so flushed all fluids and bleed. They worked great, for about 15-20 miles. Now back is spongy again. I can't see getting air in system from the caliper, I would think it's getting in from the master. Any thoughts before I rip into rebuilding it. Thanks

[Chrisj CMA]

I had trouble with my back brake too. It wasn't spongy but after changing the oil it would arbitrarily just go all the down. So I wedged something in the brake pedal over night and then bled it again and all was well. I think air gets trapped in the long route to the rear caliper

[hubcapsc]

I've never (never say never) failed to get the brakes bled using
a speed-bleeder. I've gone through a lot of fluid doing it though!

Pump. Pump. Pump... it's all clear - wait! Was that a little bubble?
Pump. Pump. Pump... it must be done now - wait! Another little bubble!

After a while no  more little bubbles. Maybe those little bubbles are the
air that's trapped in there somewhere...

-Mike

[..]

I've never (never say never) failed to get the brakes bled using
a speed-bleeder. I've gone through a lot of fluid doing it though!

Pump. Pump. Pump... it's all clear - wait! Was that a little bubble?
Pump. Pump. Pump... it must be done now - wait! Another little bubble!

After a while no  more little bubbles. Maybe those little bubbles are the
air that's trapped in there somewhere...

-Mike

[luftkoph]

This may sound goofy,I had a bike with six piston Tokiko calipers, they are know to be difficult to bleed, so once you get a bit of pressure in the brake system take an orbital sander minus the sand paper of course, and vibrate the caliper and any banjo or fitting, bleed again then use the overnight with pressure on the pedal.

[Steel cowboy]

I've had good luck with my mighty vac. Just be careful not to pump to much vacuum and suck the master cylinder dry.
https://www.harborfreight.com/mityvac-vacuum-pump-39522.html?cid=paid_google|||39522&utm_source=google&utm_medium=cpc&mkwid=s|pcrid|385364674719|pkw||pmt||pdv|t|slid||product|39522|&pgrid=79002131432&ptaid=pla-834243824145&pcid=6549872779&intent=&gclid=EAIaIQobChMI_6zxhomr5gIVFLvsCh3YGw7eEAQYAiABEgLsOfD_BwE

[Jims99]

Thanks. I will try bleeding again. I figured the peddle was good and now it's not, so the air should have been gone at first. I'll try the weight on the peddle overnight and see how that works. I'll update as soon as I can, been working late. Thanks again. 

[h13man]

When bleeding don't let the pedal/lever bottom out. Do a QUICK open/shut of bleeder when under pressure. I use gravity bleed method as I can visually control air getting into system when flushing.

[da prez]

 Reverse bleeding may be an option. I have used this method many times.
Remove The caliper and brake pads. put a thin plywood scrap or a paint stir stick in place of pads.
Be sure master cylinder is full. Pump brakes to extend pistons almost all the way out. be sure master cylinder stays above the fluid openings. Hold caliper with hose in the top position. Push pistons back in fairly fast.( Be sure not to over flow master cylinder). When done , slowly pump brake pedal a couple times. Re-install. You should be good to go.

                                               da prez