78 rear brake mess

[rhyllen]

Hi all,

I've been fighting a problem with my rear brakes for a couple of years now. I think I finally have the last problem pinned down, but I'm not sure how to go about fixing it.

Where the steel line connects to the master cylinder I just replaced the line with the closest approximation the parts store could find. The problem is, even though it seems to seal tight, no wiggle, when I depress the pedal I can hear it leaking out of the fitting.

The way I figure either I have the wrong steel line, or it's an issue with the master cylinder. Does anyone know what the specs are for that steel line? Or a good replacement master cylinder?

[whiteclad57]

So just to confirm this is with the lines empty of fluid, just air? And you are hearing this noise at the cylinder itself, not the caliper?

Is this the steel line that was replaced?

[rhyllen]

Yup, that's the line that I've been fighting for a year now. The leak I'm hearing is where that line connects to the cylinder.

[OldguyGlen]

Possible SAE. vs metric problem?? Does your parts store know the difference ?
Maybe a nick or blemish on the mating surfaces.

[Rambozo]

You probably have the wrong steel line. Some are a flare end while others are an inverted flare. You can't mix and match.
43420-371-000

https://www.ebay.com/itm/175695462473

[rhyllen]

I'm wondering if it isn't the flare that is messing me over. When I first replaced it I used a cheap flaring tool. The parts shop said that it was supposed to be a reverse flare when they sold me the line.

[rhyllen]

Possible SAE. vs metric problem?? Does your parts store know the difference ?
Maybe a nick or blemish on the mating surfaces.

It's using the same thread I think. When I replaced the original it was destroyed. I found a fitting that threaded in securely, but it could be SAE instead of metric. I'm trying to track down the size and thread of the original so I can verify or just replace with a new one. I don't really want to buy a used one to get those measurements.

[Rambozo]

From the photo on the ebay listing those are normal double flares. I would expect metric, but a thread pitch gauge can tell you for sure. Some flares are 39°but most are 45°.
If the wrong flare or a damaged flare was tightened into the master, it can become damaged and will need to be re-machined or replaced.
As the rubber hose is also not available, I would just get a new stainless hose that replaces both the hard line and the hose with one piece.

[rhyllen]

What Master Cylinder would be a good replacement? All I can find are ones that say they're for 250cc bikes other than a used one.

[Rambozo]

At this point used is probably your only "bolt-on" option.
Or you will have to adapt something from another bike.
You can start by taking your master to a good brake shop and have it inspected and tested. It might be ok or fixable.
How you fix it will also depend on if you are trying to restore it to original, or if you just want it to work.

[rhyllen]

I just want it to work at this point. It's my only mode of transportation, and I like the look of the Vetter fairing. Thanks for the help. I'll call around and see who can look at it for me.

[rhyllen]

So I'm pretty sure the rear caliper needs a rebuild. Replaced the brake line and while the brakes move they don't fully engage.

[Rambozo]

Typically bad calipers are either frozen or leaking.

[rhyllen]

Well, after bleeding the brakes they only hold the bike when it's at a complete stop. Using them while the bike is moving does nothing. They only other possibility I can think of is the new master cylinder I put on there being too small. I got one that was recommended, but that doesn't mean it ACTUALLY is the right one.

[Rambozo]

Are you running out of travel in the master cylinder?

[rhyllen]

I set the master cylinder to 'hopefully' have decent travel. I had to lengthen it to reach where it connected to the brake pedal.

[Rambozo]

What I meant was what is limiting the brake pressure?
Is the pedal hitting something?
Is the master at the end of its stroke?
Are you unable to apply any more pressure to the pedal?

[rhyllen]

I am, but it doesn't do anything. Once the pedal moves past a certain point there's no change. It feels like a set of brakes that REALLY need to be bled despite me having done so 3 times. The pedal is extremely spongy. It's not leaking brake fluid anywhere so I'm kinda out of ideas.

[cfairweather]

I probably have a rebuildable original master cylinder and an original steel pipe if you are interested. I would only use an original master cylinder and pipe in this case. If you like, I would rebuild the master for you for a reasonable price or you can do it yourself. If you do it yourself, be aware the most common mistake is not getting a very tiny hole cleaned that goes between the reservoir and the cylinder. If that hole is clogged, and they usually are, your brake will not work correctly. Do you have the original master cylinder? If so, use it, if it is rebuildable. Get a tiny guitar E string and make it go through the tiny hole. You may need to get a magnifying glass to see the hole. I use a headgear with magnification and LED lights to make it easy to see the hole. This hole must be open and the only way to verify it is clean is to push an E string through the hole.