Brake needs bled.
You've got air in the line.
When right side up, the bubble rises to the top of the reservoir. You're then just pushing fluid and the brakes feel fine.
When upside down, the bubble ends up in the caliper. So, when you flip the bike over and squeeze the lever, you've got air between the reservoir and caliper.
Simple bleed will fix the issue.
Or, just stop flip flopping the bike and go ride. You'll be ok for a while. Eventually the "problem' will get worse and you'll have a mushy lever all the time. At that point, you can't put off the bleed and will need to get it done.
You've got air in the line.
When right side up, the bubble rises to the top of the reservoir. You're then just pushing fluid and the brakes feel fine.
When upside down, the bubble ends up in the caliper. So, when you flip the bike over and squeeze the lever, you've got air between the reservoir and caliper.
Simple bleed will fix the issue.
Or, just stop flip flopping the bike and go ride. You'll be ok for a while. Eventually the "problem' will get worse and you'll have a mushy lever all the time. At that point, you can't put off the bleed and will need to get it done.
