The rear brake has stopped working on my 2014 ST.  Press the pedal and it just goes straight down, with little resistance and no braking.  I changed the pads, just in case, but no luck. 

Seals?  Master or caliper?  Any way of telling? 

Thanks in anticipation

Edited December 1, 20178 yr by stpauls

It sounds as though you’ve lost all hydrolic pressure from master to slave cylinder. Has all the fluid gone from the resivour ? 

Checked fluid.  The reservoir was still full up.

10 hours ago, stpauls said:

Checked fluid.  The reservoir was still full up.

Next thing I would check is to see if it will bleed. Do you know how to bleed a brake ? 

Replaced the master cylinder piston and seals, using a service kit, which I had bought for a previous bike, but only used the rubber shroud.  Changed the fluid and now have a good rear brake again.