Rear Master Cylinder shot?

[DSA]

Hi all,

I was riding by '76 Goldwing today. As I came to a stop at a light, I realized that my rear brake was doing basically nothing: the pedal sank to the ground but did not seem to be generating any meaningful stopping force. I tried pumping it to see if the issue was air in the system, but that didn't make any difference.
Came home and took a look, and it seems the rear master is leaking some sort of rust-colored fluid (see pics). With the pedal fully pressed, there's a little bit more resistance to spinning the wheel by hand than if it not pressed, but that's about it. I can spin the wheel by hand with the pedal fully pressed. The brake fluid in the reservoir is a normal color, not rust-colored.

IMG_1677.JPG

IMG_1676.JPG

For context, I extensively bled both brakes a few months ago, but the rear really was a mess with old fluid. My guess is that this rust-colored stuff is very old fluid, and that my master cylinder is leaking, and thus requires a rebuild.

That seem about right?

[gltriker]

Yes.

Great pictures, Jai

https://www.ngwclub.com/forum/viewtopic.php?t=2717

https://www.ngwclub.com/forum/viewtopic.php?f=8&t=50021

https://www.ngwclub.com/forum/viewtopic ... 01#p138401

[Whiskerfish]

Once the rubber around the bottom of the piston cracks it starts holding water and will cause the rust you see. Order a kit and follow the links provided above and rebuild the M/C.

[NotSoLilCrippseys]

Whiskerfish is right.

Like they did with Steve Austin of Six Million Dollar Man fame, you can rebuild it. You can make it better, stronger, faster (at helping you scrub speed).

It'll run about $40 or so for the innards. I like BrakeCrafters personally, as the kits are complete and they've treated me well when things have gone sideways. I'm sure other kits are also good.

I've done a couple rear MCs that started out seeming hopelessly rusted up. If yours is at least moving, the hardest parts of the job might be removing the MC, extracting the rusted up circlip, and getting the teensy tiny holes clear.

[DSA]

Thanks for in inputs, guys.
As usual, I went with 4into1: https://4into1.com/rear-brake-master-cy ... gold-wing/
Also ordered the big o-ring for the reservoir as suggested in that excellent writeup - had to get that separately off ebay.

Will let you know how this rebuild kit works.

NotSo: I think mine shouldn't be too bad? The rear brake was working alright up till this very last ride, so it shouldn't be jammed or anything.

[DSA]

I am 99% sure I know the answer to this question, but I am going to ask anyway because the consequences are dire if I am wrong.
I have not yet finished rebuilding the rear master. The motorcycle urgently needs to travel 25 miles. If I secure the hanging brake line (where it threads into the master) to the bottom of the gas tank, is there any reason why I cannot ride the bike?
I've drained the brake fluid out of the caliper and lines by vacuum bleeding. The brake pedal is still attached to its spring, but obviously does not do anything if pressed.

I am of course aware that maximum braking is not possible without the rear brake. But - there is no reason why the pistons on the caliper would choose to actuate and suddenly lock up the rear tire, right? I cannot imagine how with no hydraulic fluid or a piston to push it...but again, wanted to ask just in case.

[Shadowjack]

You'll be fine.

[wingrider]

I'll second the you will be fine...no hydraulic fluid to push, they won't creep out with enough force to start braking for you.

[DSA]

The urgent move got delayed a bit (rare instance where a contractor running behind schedule was good!), which gave me enough time to get the rebuild done. It went off pretty easily - I used the 4into1 kit linked above. The kit contains a spring, a piston with the secondary cup already installed, a primary cup, a circlip, a rubber cover and a small quantity of red colored grease. What it does NOT contain is the o-ring that is required to seal the reservoir to the master (I ordered one off ebay) and a new check valve (I have a 76 - in their defense, this kit is for 78-79. I re-used the old check valve.). I could not get the rod eye and pushrod apart from each other...but you don't actually need to do that anyway. The infamous circlip came out very easily, but it was hell to get the new one back on. I didn't use the supplied grease, and instead dressed everything in brake fluid when reassembling.
Haven't actually ridden the bike yet because it's already dark outside, but when spinning the wheel by hand, the pedal now stops it instantly. Low bar I know, but it wasn't doing that before. No leaking brake fluid anymore either.
Will update here if anything changes, but thus far I wholeheartedly recommend this rebuild kit. For ~$15, can't really ask for more.

[redglbx]

Good job DSA ,, the only thing I would and I don’t believe you did this was to use grease on anything that contacts anything that is brake fluid rubber as it will kill the rubber. It sounds like you lubed it with clean brake fluid ,,, again good job !

[DSA]

Yup, didn't use the grease for any part of the process. Octane's writeup I believe mentioned that, and I stuck to his advice.

[redglbx]

DSA, good job enjoy !