Issues with bleeding Shimano hydraulic brakes

[weisan]

I ran into two issues with bleeding my rear Shimano hydraulic brakes (BR-RS805)

1. I couldn't push fluid into the bleed port from the caliper after loosening the bleed screw. The cup on the shifter is opened. I have no issue doing this on the other caliper in the front following the same steps or procedure.

2. When I squeeze the shifter, it will soften at first then when I retract the lever and squeeze again, it hardens or won't move much.

What could be wrong?

[HenryA]

Have you tried adding fluid from the top? If you have a syringe and hose, you can suck from the bleed nipple while someone else adds fluid at the top. This might get you around whatever’s going wrong.

[bikinchris]

Whenever I have trouble with bleeding a Shimano brake, I use the two syringe method and push fluid both ways.
But it sounds like your master cylinder has a problem with obstruction or failed bladder. There are no parts available to buy for that.

[45K10]

I agree with bikinechris it is probably the master cylinder
I have had the same problem in the past. I tried the two syringe method and it would work for a couple of pulls of the lever and then the pistons and the lever would not move. I ended up replacing the whole brake assy include a new hose.
I really like Shimano transmission parts but their brakes are not as reliable IMO. My wife has Sram brakes on her MTB and they have been great.