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.

[isaacneff]

Another vote that it is probably the master cylinder stuck in the lever. Try adjusting the lever reach in and out to see if that allows the piston to move enough to push fluid past, sometimes that works. Otherwise, you have to rebuild the lever, which really isn't a Shimano supported service.

[weisan]

Thank you everyone for chiming in, I will tinker around a bit and report back.