I use the rear brake a lot on the sidecar, it helps with the steering a bit.  Recently it hasn't worked all that good.

If I stand on the brake pedal, I get some retardation but not much.
Syptoms: rear master cyl has no signs of leakage, brake pads show evidence of getting very hot at some stage, caliper pistons move freely.

I stripped the master cyl, stripped the rear caliper and cleaned everything(ultrasonic cleaner). 
The master cyl seals appear fine and like I said no leaks - reassemble
Cleaned and replaced all seals in the caliper, internal seals were not leaking, pistons moving freely.  Outside seals were bad.
Replaced the hose from the reservoir just in case.

Once reassembled, it's no different.  I have pedal pressure but no braking power at all.

I'm thinking that even though the master cyl is not leaking its obviously not doing much.  I know I can put a seal kit in the master cyl (the bore is good) at a cost.  There are some inexpensive master cyls available online but I cant find one that tells me the piston diameter which is the important thing.

Has anyone used one of the cheapie M/Cyls with any success?  Have a link?
Al

If the pistons are freely moving, there are no leaks and resistance at the brake pedal is appropriate, the master cylinder seems ok. Have you replaced the brake fluid within the last two years? Have you de-glazed the rotor? Doing that and scuffing the pads might change the outcome.

There are successful, less costly substitutes for oem. Here's a thread describing one of them.

All new fluid flushed through today.

Softer pads are in order to begin with, the current pads have lots of metal.  Hard wearing pads are not a requirement.

Al

Try hitting the rotor with 80 grit sand paper on an electric sander. You may have glazed the disk, sanding will rejuvenate it by removing the glaze.

n/m. I should read the entire thread first!

Al I believe an  XL600 Honda brake master is a good 13mm replacement for the rear master. Cant remember the years that fit though. In a pinch that may give you back the pressure you need, the master on your bike may have grit in the fine hole just in bore past where the piston sits its an equalisation hole and if blocked would probably drop the pressure you don't have. Maybe pull it apart and double check that little hole.

Maybe the rubber thingy that the threaded adjuster pushes is starting to perish and is squashing down instead of doing its job.

keith may be on the right thoughts Al

if memory serves me correctly rossko had a very similar issue
eventually he found the equalizer tube and freed it up ...but I think that was after he ditched the master cylinder for a newer version

Have you considered trying a smaller diameter master cylinder to increase the pressure? K bike rear master cylinders come in both 13mm and 12mm. Switching from 13 to 12 should yield about a 15% increase in line pressure.

Al, have you tried this? 

https://www.k100-forum.com/t3739-mod-to-rear-brake

Paul.

I had an issue with my rear brake and discovered inside the boot/gaiter there is a little pad/[in to go between the brake pedal and the piston on the master cylinder. This was misaligned so it was contacting the master cylinder instead of pushing down on the piston and I got plenty pedal pressure and little braking. Peel back the boot and have a look to see its all correctly aligned.