Topic: "Viscous" brake lever feel

[American Locomotive]

I've got a '76 CB750F with a GL1000 fork and dual discs up front. My problem is the brake lever feels "viscous". Here's a basic run down of what I've got going on and what I've done:

- My front brakes work well, and I have excellent braking power and can stop the bike in a hurry
- The lever is not mushy feeling. It feels rather firm aside from the "viscous" issue
- Lever effort for a given brake force seems normal. I can get pretty substantial braking with minimal hand effort
- There is no brake fade, and I can hold the lever at a given position and braking force does not decrease
- I've bled and flushed the fluid out with fresh DOT3
- I've removed and lubricated the lever pivot pin. The lever moves very freely
- I verified the caliper slides are greased and move freely
- Verified pad thickness

So on paper, everything with my front brakes is perfect. It stops well, doesn't require much lever effort, and everything is in good shape.

The problem is pulling my brake lever feels like pulling a boat oar through water. The lever will go to where I need it to go, but it will not do it quickly. It will also not release quickly. If I pull the lever all the way in, and then let go, it takes a solid second or so for it to fully release again. It's almost as if all the brake fluid to and from the master cylinder is being forced through a tiny pin hole.

Does anyone have any idea where I should start looking? My feeling is the issue lies somewhere in the master cylinder. Sticky piston, bad seals, debris in an orifice?

[Don R]

 I'd suspect bad rubber lines if they're all original or improper assembly of the master cyl.  Those are just guesses. I dual disc'd my 76 750F with the stock M/C, it had more travel but worked great.  I did put on aftermarket hoses.

[scottly]

There is a very tiny hole in the MC that relieves any pressure when the lever is released; it's the one closest to the banjo fitting. If it was partially plugged, it might cause your symptoms, but I've never heard of that happening. Run a guitar string though the hole, and if that doesn't fix it, the piston in the MC is probably dragging in it's bore, and the MC is in need of a cleaning and/or rebuild.

[American Locomotive]

Do you have a picture of what the original MC should look like? It uses the same cap as the rear MC if that makes a difference.

[American Locomotive]

The PO to the PO did the upgrade. It looks exactly the same as master cylinders I've seen on similar vintage 750Ks and Fs, but I know that doesn't really mean much as internally it could be different.

[American Locomotive]

Looked today, my master cylinder says "11/16" on the bottom if that means anything to anyone. With no lines connected, the piston seems to move "okay", but occasionally seems to slightly hang up. I did however notice the hole in the banjo fittings on all the brake lines were very small. I guess I'll get a set of new stainless lines and see where that gets me.

[scottly]

With no lines connected, the piston seems to move "okay", but occasionally seems to slightly hang up.

That's a red flag; it should move freely and smoothly without the lines connected.
The tiny hole in the MC doesn't come into play in this condition.
Your 11/16" MC is a bit on the large side, so rather than rebuild it, you might consider buying a new, smaller MC.

[American Locomotive]

Even with the dual discs? I was pretty happy with the overall lever travel distance. I'd imagine a smaller bore would make the lever travel longer?

[Bankerdanny]

Even with the dual discs? I was pretty happy with the overall lever travel distance. I'd imagine a smaller bore would make the lever travel longer?

Goldwing guru Randakk contends that the 11/16 OEM master is slightly too large. He recommends a 5/8 master for the GL100o.

"The main problem with the OEM GL1000 front master is that the relative size of the master cylinder piston to the caliper pistons is wrong. The master cylinder piston is too large and hence under-leveraged. The 11/16" GL1000 master cylinder piston operates 2-38mm single caliper pistons. Comparing the area of the master piston (239.50 mm2) to the total area of the 2 caliper pistons (2268.24 mm2) yields a master cylinder/caliper piston ratio of 9.47. This is far from ideal!"

[spotty]

just a quick one from left field here, have you done up the lever pivot bolt too tight ? it could be being clamped by its mounting on the M/C ?
i've done this and the symptoms you mention seem remarkably similar

[American Locomotive]

It's possible. I've lubricated the piston through the rubber cover, and it seems better. I will check all of the holes inside the master, and report back once my stainless lines come in.

[American Locomotive]

So whoever said to check the relief hole inside the master nailed (sort of). I tried blowing a little air through the hole, but it was blocked. So I put my air nozzle into hole where the brake fitting goes, applied air, felt a pop, and suddenly air was coming out of the hole. I then noticed the lever had moved about 1/4" further out than it was before, and the slop it previously had was now gone.

After playing around with it some more, I found that the master piston is basically binding it up in the last 1/4 of travel, so that relief hole never even got a chance to become uncovered. That explains the "viscous" feel.

Looks like it's time for a rebuild.

[American Locomotive]

So I ordered the GL1000 rebuild kit for the master and some stainless lines, and got them installed. It's working fantastic now, and the lever feel is perfect.

Thanks for the help giuys.