Topic: Replacing CB750 K2 front mater cylinder.

[colined2]

Hello all
I have a restored CB750 K2 and I only ride it every so often. The front brake is terrible. I have noticed Chinese made master cylinders that look like Kawasaki Z900 ones that come with a 14 mm piston or a 5/8" one. I have the standard single disc setup and was wondering what one to use?
Any help would be appreciated.
Regards
Colin Edwards

[bryanj]

What are you comparing the brakes to?
A standard master has been good enough for 50+ years and changing for a repop will probably make little difference

[RAFster122s]

Stainless steel discs like the SOHC4 use aren't known for great stopping power, especially with modern organic pads.

Vintagebrake.com  is a vintage motorcycle brake supplier that has a piston size to master cylinder calculator for giving you the MC to piston ratio and our stock MC tends to be pretty numb/wooden feeling with a ratio that isn't recommended. 14mm is about 0.551 inches, 5/8 inch is decimal 0.625 inches...
Stock single piston is what? 38mm
Or am I confusing that with a different bike I own?

[newday777]

Colin
You say you restored your bike.
Did you rebuild the master cylinder?
Did you replace the hoses with new Stainless Steel brake hoses?
If so,  did you bleed the system first at the master cylinder banjo bolt followed up by bleeding the caliper? That usually makes the brakes good again if you haven't yet. Just bleeding at the caliper if the master cylinder was emptied doesn't give good brake power to the caliper.

The stock system is designed for a 14mm master cylinder (9/16").
Going bigger than 14mm to 5/8" will not be productive to make better braking.
Personally I wouldn't buy the Chinese master cylinders. Getting proper rebuild kits will be a challenge later on.....

[BenelliSEI]

Colin….. I used a 14mm master from Vintagecb750.com a few months ago. It’s on our K3 for the summer and I’ll rebuild the original over the Winter. The bike has braided lines and a freshly rebuilt caliper, using Honda pads. There is no noticeable improvement over the stock master, just a nice clean, quick replacement.

Of note, most of those masters come with a plastic cap, but my original caps fit perfectly. At $66 they’re a pretty good deal too.

[disco]

Hi Colin,
I rebuilt my brake master cylinder many times. Maybe I needed to hone it, because it would always fail and I would have a dragging brake again. I bought a 14mm master cylinder from Jap Bike Spares in QLD. What a difference! My front brake is so much better. And the price was only $90 AUD!

[Kaze]

My 2 cents may have a different exchange rate on this board but...

If you have the original master cylinder, and it's not ruined somehow, it would be far better to rebuild the original with new internal parts than buying a new after-market master cylinder. If you can afford it, OEM parts are top. If not, at least stay away from no-name parts from Amazon.

Rebuilding the m.c. is also the perfect time to pop out the caliper piston, check the condition, and at least change the piston seal. Sometimes there's rust damage on the caliper piston, and that's no good. Need a bit of lubrication on the piston seal when inserted too.

I recently switched from oem lines to stainless. As previously suggested, I had to loosen the banjo bolts and bleed air out in those locations too, not just down at the bleeder. As a matter of fact, even after no air seemed to be coming out, the brakes were still awful. So I went out and bought a vaccum brake bleeing tool. It wasn't a great tool for $40, so I had one of my kids pump up the vaccum tool, and one kid pumping and holding the brake. A whole lot more air was forced out that way, and my brakes are slammin' now.

[BenelliSEI]

+1. I have an ancient “Mity-Vac” and it makes a difference.