Topic: Extremely mushy front brakes-'78 750F-UPDATE

[scondon]

The bike has taken me a year to rebuild but it's finally up and running. Problem is that it doesn't stop too well. When I pull the brake lever, the brakes start to engage at half pull but the pressure never builds and the lever hits the handlebar without ever getting stiff. What I have done so far:

1) replaced brake lines with steel braided ones

2) rebuilt the Master cylinder

3) put a plug in the MC and pulled the lever. Hard as a rock so MC is good

4) put a MC from another bike that is known to be good. Mushy brakes

5) put a plug in right caliper line to isolate the left caliper. Mushy brakes

6) put a plug in left caliper line to isolate the right caliper. Mushy brakes

7) Sucked a 1/2 quart of brake fluid through the system. Mushy brakes

even bled the little bolt at the top of the line splitter. Mushy brakes

          OK, so the calipers are coming apart tomorrow but I don't know what I could possibly be looking for that would be causing this. They were rebuilt three years ago and I am not losing fluid anywhere in the system. Brakes act like there is a golf ball sized pocket of air in the system. Any ideas?

[Dennis]

Brakes act like there is a golf ball sized pocket of air in the system. Any ideas?

There's not a golfball sized pocket of air in the system, but I'll guess that there are a few (or maybe a few dozen) tiny little air bubbles. I'm not familiar with your specific model with the dual calipers, but what I would try would be to pump fluid into the calipers and try to push the air out of the top at the m/c. That's the way I try to bleed all of my bike brakes.

[seaweb11]

dito, you probably still have some tiny bubbles left in the line.

[Bill Vaughan]

Sounds like there's still air in there somewhere.

Just did the same thing on a '78 Harley (sorry) tonight after rebuilding a leaking master cylinder.  One thing that helped:  used different color brake fluid so that I was sure that the old stuff (and air) was out of the system.  Oringinal DOT 5 was yellow; replaced it with DOT 5 that was purple.

[scondon]

          Thanks for the quick responses fellas. I bled the brakes many times using the traditional method (pump lever,crack bleed valve, tighten, repeat). And I've bled them repeatedly using a vacuum bleeder. The last time I bled them I pulled a whole bottle of brake fluid through the system with no change whatsoever. I've also zip tied the lever to the bars overnight before bleeding.

       I've got another bike, same year and have not had any problems bleeding the air out and the brakes are rock solid. Any other ideas?

[mick750F]

   I recently installed stainless braided lines and rebuilt the mc and calipers on my '78F. It took a long time to get the system bled using the traditional method. I'm guessing you still have some air in there somewhere. Maybe at the splitter? Maybe you could try tapping the calipers/splitter/mc as you bleed? Wish I could offer something else...I guess I'm as stumped as you.

Mike

[MikeDeB]

          Thanks for the quick responses fellas. I bled the brakes many times using the traditional method (pump lever,crack bleed valve, tighten, repeat). And I've bled them repeatedly using a vacuum bleeder. The last time I bled them I pulled a whole bottle of brake fluid through the system with no change whatsoever. I've also zip tied the lever to the bars overnight before bleeding.

       I've got another bike, same year and have not had any problems bleeding the air out and the brakes are rock solid. Any other ideas?

I've done dual caliper systems before with no problems.  You've got to stroke the lever fast and build pressure to get the air bubbles to move to the bleeder.  Do one side then the other and repeat.  Remember, air can't get trapped in the system if your using the correct bleeding method.  Why?  Brake fluid, unlike water, has virtually no surface tension so it will not allow an air bubble to cling to anything in the system.  Ever notice how brake fluid goes everywhere when spilled?

[scondon]

      I've bled the MC, both calipers and the little bolt at the top of the splitter. I've bled the system numerous times over the course of a few days. I've used a bottle and a half of brake fluid without seeing a single air bubble come out. Usually, at some point during the bleeding process one can feel the brakes start to "stiffen". In this case it never happens. The lever begins to build pressure to a certain point and then fades as if fluid is escaping somewhere, but there is no fluid escaping anywhere. I'm not saying it's impossible that there is ANY air in the system but I'm sure,at this point, it is not the source of my problem.

      Having rebuilt the master cylinder and confirmed that it is good, I am now going to take the calipers apart and see if anything crawled in them and died during the year they were off the bike and in a box. I stored them with the banjo bolts threaded in so I can't see how anything could have gotten inside though.

        I'm remembering an old grade school science class where we added water to corn starch and squeezed it in our hand. It would become solid under pressure and when the pressure was released it would revert back to liquid. My only remaining hope is that there may be something trapped in the caliper bodies that is behaving simiiarly.

        I can pull the brakes off of my other bike and install them on the rebuild. Then start swapping the parts one by one with the brakes that aren't working until the problem repeats itself. But this will take a lot of time, get even more brake fluid on my newly painted stuff, and leave me without transportation (both bikes down). So here's hoping someone else has experienced something like this that wasn't attributed to a faulty MC or air in the lines.

[MikeDeB]

It sounds as if you have an internal seal giving way under pressure (in the MC) since the lever builds pressure then acts as if pressure is being relieved but no fluid is coming out of the system.  It won't be a caliper seal, otherwise you'd have fluid all over the floor.

[scondon]

          Even though I rebuilt the cylinder and tried another MC from another bike already, I am not above swapping the MC from my daily rider. I'll make it the first order of business this afternoon before pulling the calipers apart.

[Chris Liston]

Sounds like there's still air in there somewhere.

Just did the same thing on a '78 Harley (sorry) tonight after rebuilding a leaking master cylinder.  One thing that helped:  used different color brake fluid so that I was sure that the old stuff (and air) was out of the system.  Oringinal DOT 5 was yellow; replaced it with DOT 5 that was purple.

Just a thought but purple DOT5 is synthetic.  DOT 5.1 what you probably had isn't.  Just make sure you completely flush the old fluid out.  You probably did that though. 

[mick750F]

It sounds as if you have an internal seal giving way under pressure (in the MC) since the lever builds pressure then acts as if pressure is being relieved but no fluid is coming out of the system.  It won't be a caliper seal, otherwise you'd have fluid all over the floor.

   MikeDeB could be onto it here. I remember now when I rebuilt mine the manual suggested NOT letting the lever come all the way to the bar when bleeding the system. There is a possibility of damaging or unseating a seal.

Mike

[eldar]

CHeck your lines themselves as you pump the lever. Check for any leakage, check for cracks.

Try this experiment, Put your master on your OTHER bike and while it may not be strong enough to retract the pads, it should still push firmly. If that happens, you can rule out the master then. While that my not be a huge help it is something. I did not remember reading thins but if you pump the brakes do they firm up some? If so then it is probably not a leak and somewhere air it hiding out and that happens in the calipers and the joint between the brake lines.

[scondon]

  After 3 hours of switching brake parts and taking the calipers apart for cleaning and inspection I finally tried taking the brake lines from my daily rider and put them on my troublesome rebuild. BINGO! Brakes are rock hard and it didn't take much bleeding at all to get them that way.

  The brake lines that work are two long lengths that go directly to the calipers, bypassing the splitter. The ones that don't work have one short length going to the splitter, then two lines going from the splitter to the calipers (stock configuration).

  At no time during the long process of trying to get the brakes working with the new lines (stock configuration) did ANY brake fluid seep from them or anywhere else, and I am absolutely certain that there was no air trapped in them. Yet the brakes acted just like a faulty master cylinder i.e. pull the brake lever and it would build slight pressure and then fade until the lever hit the bar. I kept expecting to see a thin stream of brake fluid squirting out somewhere but it never happened. WTF

  I already plan on pressure testing each of the new lines using a MC and a caliper with a straight(non-banjo) bolt but I don't really know what I expect to find since they never leaked. I'm getting a bald spot from all the headscratching over this one(haha) and am open to any and all theories that don't include air in the lines. Up to and including gremlins, poltergeists, and goverment conspiracies.

[bryanj]

There is a remote possibility that the hydraulic brake light switch on the splitter has developed an internal leak. I have never had one that didnt pee fluid out of the cable connections when this happened but i theorise that it is possible.