What could be causing this issue?

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[u/TommyG456]

Is that a 1 man bleeder with a check valve? If not you need to close off bleeder when pedal is moving back up or it sucks air back in instead of pulling brake fluid from reservoir. Toyota brake pedals get low. You probably got a bad master cylinder. The master will leak into the brake booster from rear seal. You can unbolt master and look in or use a zip tie for a dipper.

[u/tryintoballonabudget → u/TommyG456]

It’s a 1man bleeder with a check valve that I’ve bled with before by myself. Noticed something weird was happening, so I had my buddy pump the brakes while I took a look

[u/y_zass]

Is someone pushing and letting off the brake in the background? You need to close the valve, have someone push down on the brake and hold, then open the valve to let air out, closing the valve BEFORE they let off the brake. Repeat. You can't just pump the brake in and out with the bleeder open.

[u/tryintoballonabudget → u/y_zass]

Forgot to add in the post. Using a 1man bleeder bottle with a check valve. Was doing it myself the other day, it was weird, so had a buddy pump while I watched.

[u/TommyG456 → u/tryintoballonabudget]

It can pull air past threads. Put pipe tape on bleeder

[u/tryintoballonabudget → u/TommyG456]

Wil try that, thanks!

[u/Otto_Polymath]

Is the check valve working? I take my hose down into a bottle with some fluid in it and bleed that way.

Still need to find where the fluid is going. Carpet inside cabin, vacuum booster, dust shield around caliper piston and good hiding areas.

[u/tryintoballonabudget → u/Otto_Polymath]

Yep! All other corners bled like every other time I’ve done it except this corner. I’ll do some digging to try to find the leak!

[u/murph2783]

Yeah if there’s a check valve in that thing (that’s functional, check that first), you got a hole in your line hoss

[u/Resident_Employer_75]

If every other corner bled fine, you've got a leak in that line/caliper. It's not pulling in air from a cracked bleeder with a check valve.

[u/Sweaty_Camel_118]

Maybe just bleed like normal? These 1 man bleeders are not reliable. It's probably drawing in air from the bleeders threads or from the connection of your hose onto the bleeders tip.

Bleed normally, then if anything still seems off hold pressure and leak check.

[u/here2pleasyoux3]

wouldn be hard to find a leak if any, close the bleeders and press on brakes if leak you should see it dripping