Check relief port in M/C is open. Remove filler cap and insert fine wire in smallest hole. I am assuming piston returns properly when you release the pedal.This activity you can see in the M/C when you shine a torch inside and with a helper to push the pedal slowly. You can also remove the fluid with a medical shringe while you are there and fill with new fluid. Use paper towel inside the M/C to mop up the dirt inside.
Dont push pedal while you clean the M/C out.