Topic: '78 CB750F2 Front Brakes

[Darrell]

So my front brake lever started giving a lot about a month ago - like coming in all the way to the handle bar sorta giving. I tried draining the fluid and refilling/bleeding, but it didn't help. I assumed it was the brake lines (they were original after all), so I ordered new Galfer stainless steel replacements. After putting them on last weekend, I found that didn't help at all either.

I rebuilt both calipers and the master cylinder when I got the bike just shy of three years ago, but for kicks and giggles, I popped the cylinders out of each caliper, and despite looking pretty clean already, I cleaned them and the dust boot and reset them. Filled up the fluid, bleed, and still no go.

I had a hell of a time getting the c-clip thingy back on when I rebuilt the master cylinder, so I'm cautious of pulling that apart again, but I have no idea what else I can check that's causing the problem. If I take one of the lines off the caliper, plenty of fluid squirts out when I squeeze the brake handle, so that's another thing pointing to the MC being OK.

I'm pretty stumped here. The braking system isn't the complicated, and everything seems and should be working perfectly... but it's not. Any ideas?

Thanks.

[bryanj]

If the lever comes all the way back and there is no fluid leak anywhere it has to be a master cylinder problem like a seal leaking back under pressure.

[Lumbee]

...your next option would be a new MC...I think partsnmore has them for $50...

[jay]

i worked  on auto brakes and what i used to do to check mastercylinder  was block all the flex hoses (rubber lines at each wheel in front one in back at year end. there are actual tool for this procedure that crimps the hoses without damaging hose but vise grips will work just as good. anyway what i am trying to say is this should work for bike as well. so gently withvise grips  (may need 3 or 4 pair. clamp off all hoses on bike. pull lever if there is no change it is mastercylinder. if it feels good there is trapped air somewhere in line may need to bleed some more.                                                hope this may be of help.     jay

[Darrell]

So I pulled apart the front MC and it all looks fine. Like I said, I ordered and installed a rebuild kit for it about three years ago now (which included a new spring and seals, and I think piston).

Any ideas what I should check for? I don't really want to order another rebuilt for the hell of it.

Thanks.

[jay]

ok  if you really think mc is ok and there are absolutely no leaks,there is definitely air in system somewhere. a mc on a bike doesn't have much travel so it can take long time to get trapped air out. i suggest you to put bike on centre stand take cover off mc and make sure topped of with fluid. leave cover off and crack open bleeder on caliper. now just let gravity bleed for a while, make sure you don't let mc go dry. this could take a while so be patient. (put something under caliper to catch fluid).  after you let the equivalent of approx 2 full mc Worth of fluid go threw system, close caliper bleeder. top off fluid put cover on and try the lever.  doing this procedure is actually doing 2 things getting trapped air(if any) plus flushing system and putting new brake fluid . brake fluid absorbs moisture and needs to be flushed more often then people actual  do, once brake fluid absorbs to much moisture the brakes systems boiling point decreases.  if braking really hard rotor gets hot , caliper gets hot fluid gets hot . if it boils  air gets in system . (make sure you only use brake fluid absolutely nothing else because all rubbers will swell ruin your whole brake system.if this happens you will have the same prob you have now. try this  and let me know how you made out.

[super pasty white guy]

You could also try a vacuum bleeder setup.  I bought one this year to do my car's brakes but it works a treat on the cycle as well.

Harbor freight has them for less than $20.

dave