Ml350 2010 spongy brake pedal

[Te2800]

Have my ml350 2010 and when pressing brake pedal it goes straight to the floor it stops ok but doesn’t feel right ..if I try and pump pedal it makes just a little bit of pressure but pretty much still sinks to the floor .. local garage fitted one rear calliper ad said it needs another ? But all seems to be working ok any help here please and is it normal oressbrakecon all four corners to bleed system it seems to work for a short while then problem persists again