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Yes, I'm still fighting to get a rear brake.
Having decided that the original master cylinder was the cause of the no pressure problems, I finally managed to get another decent 79 one for rebuild.
So this is where I am now. Rebuilt master, rebuilt calliper, new stainless full line.
Pressing the brake pedal produces big bubbles in the outflow container. The reservoir fluid level does not drop.
There is no visible sign of fluid disturbance in the reservoir, which you would normally expect, when the pedal is pressed.
I have tried sucking the fluid through. Much sucking, much air and no fluid. There are NO fluid leaks anywhere.
I'm at a complete loss. Never had anything similar happen,
Ideas, please???

Edit: just had the thought that, unless the pedal was down, fluid couldn't be sucked though anyway, could it?
Going to give that a try,

Have you checked the "splooge" hole open?

Hey John,
To me, this feels like the student helping the master . I would start with disconnecting the brake line at the master to see if the master is building pressure. Then reconnect and do the same test at the end of the brake line. See where this is going?
Is there any chance the internal parts of the master are in the wrong sequence? I once had a front master that refused to build pressure. Hope this helps.

It's midnight and this is driving me NUTS!
So I go back in the shop and take the master off, again. I had the thought that I may have put the return spring in upside down, which would have forced the lips of the top seal against the bore walls.
Stripped it down and checked everything again. Nothing apparently wrong. I had made a point of opening the splooge hole when I first cleaned everything.
Reassembled it, mounted in the vice and some fluid. Pumped the rod and guess what................NOTHING was squirted out. Not even a drop.
That simply shouldn't be possible. Even if the bore was worn and the seals were not doing a good job, it was pumping against no resistance so there should have been something. But if the wear was there, there would be leakage past the seals and there isn't any. I have no answers.

Edit: Tomorrow I will use the same internals and rebuild the old body. if I get the same result, it has to be the new seals. I bought the kit from David Silver, don't remember the make, one of many parts I have bought from them without any problems.
==

At least you found the problem. Just put another one on

78/79 masters just don't exist over here, Doug. Getting that one took weeks.

I guess if I'm forced to, I'll have to use an early one. They are slightly more available.

good point. If the bore is good a new kit maybe

Is the plunger fully returning to its normally retracted position?
I'm not familiar with a GL rear master cylinder. (My "brike" doesn't have a rear master cylinder.)
I recall with my 1975 GL1000 front brake master cylinder, if you slowly and slightly squeeze the brake lever during a bleeding session, you will see a very small single bubble of air appear in the brake fluid reservoir. That air bubble exits the compensating port aka the sponge hole.
Bench bleeding the master cylinder is a Best Practice and may be the answer to this perplexing quandary. Maybe yes... Maybe no

Is there really that much difference between the first, second and even the third gen models?

The physical mounting of the MC changed from 77 to 78.

One thing to check John is the return spring. When we were working on Liam's bike we wasted a whole day it seemed on the back brakes. The return spring in the rebuild kit was just a hair shorter than the original and the plunger was blocking the spooge hole. It was very frustrating. I know we had that M/C off a dozen times.

Well, back to it. Stripped the 'new' one and rebuilt the original. I had already measured all the parts against the originals, Steve. Couldn't find any differences.
Mounted in the vice, pumped the piston and same result, nothing.Just standing there, wondering what the hell was the next step, I gently pushed the piston rod. Oh, wait, fluid!!
Then I realised just what an idiot I had been.
When you press the brake pedal you only squeeze it, not pump it. Due to the design, pumping causes a sort of self defeating action. A couple more squeezes to prove it and it was ready to go back on the bike.
But I knew there were other problems due to the amount of air when I was sucking it, so I redid all the connections, including annealing all the copper sealing rings.
Bleeding this time was fairly swift.
I HAVE A BACK BRAKE at last. And a red face.

ritalz wrote: ↑Mon Jul 14, 2025 11:06 pm Is there really that much difference between the first, second and even the third gen models?

This is a list I did some time back for one of the FB forums members who wanted to know what the differences were.
This is the changes from 77 to late model.
From front to back:

Front wheel
Axle spacers
speedo drive
fork sliders
lower fender bracket
brake caliper brackets
78 calipers, again changed for 79
78 rotors, again changed later
Brake switch delete on splitter. Moved to master cylinder
Splitter cover with HONDA badge 78/79
Master cylinder square 79
Black plastic headlamp bucket
Headlamp brackets
79 oblong indicators (only some regions) KZ
Switches colour changed. Green lettering.
Clutch lever separate from switch body
Speed and tach faces colour changed (twice)
Tach red line dropped to 8k
Temp gauge deleted from tach
Shelter and shelter panels
Logos on panels
Extra instruments in panel on shelter
Shelter centre panel vinyl covered
Wiring harness different.
Frame changes to mounts for shelter and side panels
Frame strengthening
Carburettors
Camshafts
Auto advance unit
Crankcases. Second change, first one in 76
Rear cover kickstart delete
Primary chain sprung tensioner fitted
Improved gear bushes
Main bearing bridges and bolts design changes
Cylinder head bolts change to spiral type 79
Strengthened gear selector forks
Heat shields behind timing belts deleted
Different tensioner support brackets
Headers
Chrome covers on headers
Chrome silencers
Side frame kick stand mount to accommodate the new exhaust
79KZ (only some regions) had improved swing arm bearings
Strengthened U/J on drive shaft
Different battery box
Different air cleaner and hoses
Rear wheel
Rear brake caliper
Rear brake master cylinder
Rear brake light
Oblong indicators (different regions had different rear lights and indicators)
Seat slightly wider and longer.

I have no doubt I have missed something.

I was trying to find the Factory Service Bulletin, that I know I have somewhere, noting the four different clutches between 75 and 76.
The only thing constant with these bike was constant change!

There were indeed many changes. My comment was directed towards changes to the rear master. Glad you got it sorted.

ritalz wrote: ↑Tue Jul 15, 2025 10:16 pm There were indeed many changes. My comment was directed towards changes to the rear master. Glad you got it sorted.

As WF noted, the body of the 78/79 changed due to the frame changes. There was also a change in the internals; the 78 still used the check valve, the 79 does not. I don't know why it was eliminated; I assume the port drilling changed but I don't know in what way.