Topic: Raining Brake Fluid on a 76 750K6: Is It Time for a Master Cylinder Overhaul?

[cshanek]

Greeting grizzled SOHC vets,

I’ve been dealing with a rather annoying brake fluid leak on my 1976 CB750K6 for quite some time. While riding, even without using the front brake, drops of brake fluid run down the top side of the brake lever and eventually end up on my leg, handlebar switches, or occasionally the tank. Initially, I suspected the crush washers and banjo bolt, but after replacing those, they seem to be dry after a ride.

Now, I’m thinking the issue might be with the seal in the master cylinder (the one the brake lever contacts). I have a few questions:

• Is this a common problem?
• I’ve found many reasonably priced master cylinder rebuild kits. Is this a straightforward fix, or would you recommend purchasing a new master cylinder, which appears to be about 2-3 times the price of the rebuild kits?
• What is the purpose of the plate shown in the attached picture? Are there any issues with it not sitting completely flat?
• Relatedly, what are the torque specifications for the brake lever bolt and banjo bolt? I haven’t been able to find them in any of my shop manuals.

As always, I appreciate the help and pointers.

[bryanj]

Go to Dave Silvers site and look up the cost of a repop mater, changing the seals is a bugger as the circlip is a swine to get out and to be honest if its actually leaking the bore is probably damaged any way.
Attached should be a screenshot from the uk silvers site

[Don R]

 Bryan is right about the circlip, be prepared to buy a proper tool to wrestle it out. The normal parts store snap ring tools usually are too short. I've rebuilt some pretty gnarly looking M/C's with no serious issue except getting them apart.

[Don R]

 And, don't pull the lever back too close to the grip, the seal can travel too far and cause fluid to pass it causing it to leak out of the dust cover.

[HondaMan]

That metal plate on the bottom of the cylinder holds a short piece of rubber hose in a hole in front of the brake lever, to prevent its slack bouncing around, and to slightly hold the lever pressure against the master plunger when the brake is not in use. If the master cylinder is leaking from the lever's piston, it will leak out there. This slight pressure on the lever was considered a 'constant bleed' approach in case air developed (somehow) in the brake system's line. The early bikes had a complex offset cam, washer and locknut there instead of the rubber hose cushion piece, that's how important Honda considered this little item to be. In actual practice, the rubber hose/rod in the hole worked as well, and didn't require tedious setting of the position after a rebuild.

[PeWe]

The repop from david silver work fine. My K6 has  used one since 2015.   Soon 60.000 km. Dual stock brakes most of the time.

The cap looks too cheap though. I ordered a better looking with the mc.
The upper cap for my bikes

Here the UK site
https://www.davidsilverspares.co.uk/CB750K6-FOUR-1976

[HondaMan]

Yeah, Per: mine still has the "Use J1703 Fluid" logo on the top, too!

[cshanek]

Might not be the best idea, but since the prices were so low I decided to do both. I bought these on eBay ...

• https://www.ebay.com/itm/203199822507
• And hope to tinker and rebuild my OEM master cylinder with this rebuild kit: https://www.ebay.com/itm/352927654738

[PeWe]

I have a restore kit for my old K0 MC my K6 used in the 80's. Stock broke when I dropped the bike early 1983. A minor crash...
A friend found a replacement. It started to leak when bike came out on the road again 2014. In a barn since fall 1990.

I'll use the Honda rebuild kit later on. The K0 or older mc might be useful.
Just find all other parts for a complete bike

[carnivorous chicken]

I've had good results from the DSS replacements, bought two and no issues. I've also rebuilt them.

If you're rebuilding the old one I've found the easiest way to wrestle that clip out is with a dental pick not the snap ring pliers (most are too short anyway), being careful not to do any damage. I've had those clips break in the process as well, which isn't such a bad thing in terms of getting them out.