Topic: Disk brake driving me mad

[jimbojangles]

Hi. I'm new to the forum, but I've researched this problem every way I can think of.

I bought a '73 CB750 a couple months ago and have finally got it running again. The problem is the stupid front disk brake. I've rebuilt the caliper no less than 5 times now and it still drags after only a few squeezes. After a rebuild it will work fine for a short trip, but then it starts to rub and the handle will become extremely stiff to pull.

What I have done:
Pulled caliper apart, cleaned inside, greased and reassembled. (Rebuilds 1-3)
Pulled caliper apart replaced seal in caliper after cleaning the groove until it was spotless (rebuild 4 &  5)
Adjusted bracket screw
cleaned return hole in reservoir
bled entire system

Any ideas would be a huge help.

Also, Is it necessary to buy a mc rebuild kit if you are just going to take the mc apart to make sure everything is clean? 

Thanks

[flybox1]

are your brake lines original rubber, or new braided stainless?
just a thought, if they are the original rubber they may be soft, caving, and not letting fluid return to your MC to retract your piston.

no need to buy another rebuild kit.

[eurban]

Just to make sure here . . .There are two holes in the bottom of the master cylinder resevoir, one being a tiny hole small enough that a sewing needle is about the right size for cleaning it out.  When this smaller hole is blocked your type of symptoms are the result . . .

[jimbojangles]

I have the original lines, but they are usually pretty hard with pressure when this happens. I'll double check today anyway.

I've cleaned both holes with a guitar string. I shot fluid all over the tank when I pushed on the caliper the other day while the cap was off the reservoir so I know that some fluid is able to make it back. That wasn't a good day!

I haven't rebuilt the MC yet because I wasn't sure if that's where my problem really is. I want to take it apart to have a look, but do I need to purchase a rebuild kit first?  Money is tight right now, but reliable brakes are most important.

[Nikkisixx]

are your brake lines original rubber, or new braided stainless?

I have the original lines,

I think flybox is onto something.  You are asking a lot from 40 year old rubber lines, they could be your problem.

[HawaiiMike]

Changing the MC with one of those on E-bay saves time too.  They cost about $50-$60.  I have one and it's great.

[flybox1]

yes, they will get hard with fluid pressure when you pull your brake lever.
im just saying if they are at all 'soft' from age, the walls wont be strong enough to hold shape while your MC request fluid return.
it might not manifest as a collapse in one localized area.  it could be weakness throughout the whole breakline.  just enough all over to not be visible.

[Pirate Gonzo]

Jimbo,

I'm not much help but I have been dealing with the same thing for years. I have a duel disc set up on the front of my 550 and one brake drags. I think my problem is the piston is scratched and not allowing it to suck back in.

[madmtnmotors]

Don't overlook the caliper bracket pivot (where it mounts to the lower fork). If the pivot pin is near siezed in the bracket you might experience these symptoms. Make sure the braket pivots freely.

[jimbojangles]

Thanks everyone.

I took both of the brake lines off and blew them out. The bolts that connect to the triple tree we pretty gummed up. It seems to be working a little better but it always does for about a day until it gets too much pressure built up.

I have new pads coming in the mail as a last ditch effort. I thought maybe the extra pad width could somehow help the cylinder retract. Either way pads can't hurt because I'm pretty sure they are 20-30 years old and make an awful squeal.

Is there a good place to order brake lines for our bikes? I haven't seen them on anywhere and my local Honda dealer is pretty expensive and treats you like dirt unless you buy a new bike. My caliper seal was almost $15.

[mgbgt89]

A member on here makes braided stainless hoses. Pampadori i think is his name.

[Kevin400F]

Jimbo....

One diagnosis technique that I haven't seen mentioned is cracking the caliper bleed screw when the "pressure buildup" / excessive brake drag symptoms are observed.  It you get a spurt of fluid and the brake releases, then you know you have some sort of issue with the hydraulics....either a brake line interior has failed and is acting like a check valve, the recouperation hole in the M/C isn't functioning, or the brake lever is incorrect masking the recoup hole. 

If when the brake drags, cracking the bleed screw doesn't help, then you must have a mechanical drag....either from a sticking piston, sticking pad or binding caliper arm pivot.

Good luck in your troubleshooting.

[camelman]

Jimbojangles,

One very common mistake when mounting the front wheel can cause this.  Check the nut on the front axle to make sure it is installed correctly.  I have not worked on a CB750 yet, but I have worked on most of the other 70s CBs, and they are all set up with the flats of the nut on the inside of the fork leg, not the outside.  If you mount that nut with the flats on the outside, then it will set the rotor crooked in the caliper and the brake will drag.  New pads will only make it worse.

Camelman

[Spanner 1]

Guys, stiff handle can only be  caused by fluid not returning to the master cylinder when the lever is released... common cause, return hole in the m/c blocked ( the teeny one). Are you sure it's open ? Can be stubbornly blocked. But don't open it bigger than it is.. i.e. use the smallest needle in yer Mom's sewing box ( only ).......

[jimbojangles]

Thanks guys. I think there's extra pressure per Kevin's diagnosis. That's why the lines are rock hard even when I'm not squeezing the brake.

I'm pretty sure the return hole isn't blocked because plenty of air bubbled through it when I bled the brakes. It also sends a small amount of fluid back up when I both squeeze the brake and when I let off. Could this be part of the problem?

How far should the guitar string go when I try to clean the hole?

[Waggles]

Is it possible that the wing mirror has been tightened down too far and is binding on the brake lever?

Just a thought

[hondaface75]

Just sharing my experience, once I had installed new pads on my 75 750K and the 'stay' screw that goes into the top of the brake pad on the caliper side wasnt in far enought. Holding the pad out at the top all crooked was making my brake drag. Probably not your problem, but just a thought to check and make sure the pads are aligned and installed properly.

[333]

Rebuild the master cylinder.  The kit is around $12 plus a pair of needle nose snap ring pliers.

[flybox1]

www.slingshot-cycles.com for your SS brake lines.