Topic: 1976 CB750 Front Brake Problem

[Truiji17]

Hi There

I need some help figuring out a mystery on my 1976 CB750 Four.

The front brake works just fine. When riding the bike, after a while (3 or 4 minutes) on the road without using the front brake, the first time I try to use it, it feels like there is no brake fluid in the cylinder but, after 1 or 2 pumps, the brake responds normally and works as expected again.

When I use the bike after a few days in the garage, the brake feels perfect. After I start riding.

My working theory is that the engine/road vibrations make the master cylinder empty.

There are no leaks, and the brake fluid level does not go down.

So far I have:

Rebuild the original front bake master cylinder. Twice.
Replaced the entire front brake master cylinder assembly.
Replaced front brake caliper.
Flushed the circuit completely.
Purged the circuit.

The only part of the system I have not touched yet is the actual brake lines. They are NOT original and steel-braided.

Any ideas or suggestions?
Thanks in advance.

[newday777]

Welcome to the forum
You're reply wasn't real clear on the brake lines, are you using new stainless steel lines or rubber hoses?
Your words were
"They are NOT original and steel-braided."


Did you bench bleed the master cylinder at the master banjo bolt before bleeding the rest of the lines?

EDIT..... ADDED TO CLEARIFY....
This needs to be done when rebuilding any master cylinder that is empty, as air gets trapped in the master cylinder that won't bleed out the lines to the caliper if you don't use the banjo bolt as a bleed nipple point first(use a rag to catch the brake fluid that escapes when cracking open the banjo bolt as there isn't a nipple there to attach a catch hose to). Pump up pressure and hold lever, crack open banjo boltto release air bubbles and fluid. Tighten bolt before you release lever. Do this several times to ensure all air is out of the master cylinder, then bleed the banjo bolt at the junction on the triple tree where the 2 hoses come out of the pressure switch block, followed with bleeding at the caliper. Doing it this way ensures all air is removed from the system and doesn't trap little bubbles in the lines causing problems on the road.

[RAFster122s]

Rubber brake hoses should be replaced every 5 years but rarely are. Odds are you are experiencing collapse internally of the brake hose...they act like a one way valve sometimes. You aren't seeing that failure mode yet, or once you applied brakes they wouldn't release. Stainless Steel Teflon lines are best. Apex Brakes is good source for new lines. Drop them an email with model specifics and as long as your bars are stock and MC then stock lines are what you need. If MC outlet is different or bar height changed then you need to adapt upper brake hose for correct clocking of the line and length.

[Don R]

  Wow you did your due diligence so far. Try pumping it up and leaving a couple rubber bands holding pressure overnight. Tap the lines and M/C with a screwdriver handle or similar tool to bump any small air bubbles loose and then see if you get air in the M/C when you release the lever and pump it.

[RAFster122s]

Did I misread? Do you currently have stainless steel Teflon brake hoses? If so my internally collapsing hoses does not apply...

Then your MC is likely trapping a bit of air. The rubber band trick helps small bubbles escape overnight.
Do as Don and newday suggested.

[bryanj]

Do you have the adjuster screw correct and are your front wheel bearings good one or both of those could cause the piston to be pushed back when riding

[newday777]

Note the added edit to my earlier post.
Truiji17... and be sure to reply back and not leave your thread hanging unfinished.

Welcome to the forum
You're reply wasn't real clear on the brake lines, are you using new stainless steel lines or rubber hoses?
Your words were
"They are NOT original and steel-braided."


Did you bench bleed the master cylinder at the master banjo bolt before bleeding the rest of the lines?

EDIT..... ADDED TO CLEARIFY....
This needs to be done when rebuilding any master cylinder that is empty, as air gets trapped in the master cylinder that won't bleed out the lines to the caliper if you don't use the banjo bolt as a bleed nipple point first(use a rag to catch the brake fluid that escapes when cracking open the banjo bolt as there isn't a nipple there to attach a catch hose to). Pump up pressure and hold lever, crack open banjo boltto release air bubbles and fluid. Tighten bolt before you release lever. Do this several times to ensure all air is out of the master cylinder, then bleed the banjo bolt at the junction on the triple tree where the 2 hoses come out of the pressure switch block, followed with bleeding at the caliper. Doing it this way ensures all air is removed from the system and doesn't trap little bubbles in the lines causing problems on the road.

[MauiK3]

+1 on tying the brake handle back overnight, works great.

[Truiji17]

Thank you all for the help…
To clarify, the bike has new steel-braided brake lines.
I will try holding the brake lever overnight and report back.
Stand by…

[Truiji17]

Mystery solved

I was getting ready to leave the brake under pressure overnight and taping away every part of the system when I noticed the caliper adjuster/stopper lock nut has been lost and the stopper screw was way off and almost falling apart.

It seems with the stopper bolt off, the caliper cylinder was retracting too far away and pushing brake liquid back to the reservoir.

After replacing the nut and a quick adjusting, the problem is now resolved.

Thanks again everyone.