Shimano hydraulic brake loses pressure I recently changed the brake pads on my Shimano 105 (ST-R7020), and when pushing the pistons back, I felt a "pop", and the pistons moved back with no resistance at all.

After using the bike for a couple of days, the brake now does not work anymore. It has become very "spongy" and does not maintain the pressure. I can pump it again, and then it keeps pressure for a while, but then soon loses pressure.

I have tried bleeding it again, without success. I fear that something is broken in the brake lever - something that would normally prevent the spring between the brake pads from pushing the oil back in the reservoir.

Since a new brake lever is expensive, I was hoping someone would know how to fix it? Or know where to get spare parts for the brake lever, instead of having to buy a completely new.
I resolved the problem by changing the caliper. Now everything is working fine. Thanks for all the help, and details on how a brake system works.