Sportster front brake madness... Help!

[DrSweden]

So fed up. Bought a used 89 sporty with converted fork to longer and wider. Brake system is OEM sporty 89. I noticed the rotor was warped, had a mushy pull, so i bought a new only to find out bind (which I assumed lead to the warp). I have bought a new (used) lower brake cylinder, new brake line, cleaned the system and added 5.1 flued instead of oem 5.

I take a century to fill and blead, i pull the brake handle like an epileptic, hold, bleed, like hundreds of times, I have to push in fluid in lower cylinder to even achieve some build up of pressure. When I finally get some pressure/resistance the cylinder moves pretty weak and the starting point, like dead zero of cylinder automatically leads to the bind.

Can master cylinder be faulty? I see how cylinder moves a few millimeters...

[JBinNC]

First, you should not use dot 5.1 in that system. It is designed for dot 5. Dot 5.1 is like dot 3 or dot 4. I don't know why someone screwed up that designation so bad. Morons running the world now, I guess.

Second, it sometimes takes a long, long time to bleed these front brakes. If the system is empty, I will fill the master cylinder, open the caliper bleed screw, and walk away, letting gravity do the work. Keep the master cylinder reservoir topped off. Then close the bleed screw and carefully pump the lever, which will bring air bubbles up to the master cylinder. That part can easily take 15 - 20 minutes.

Third, you MUST have movement of the caliper on its mounting pins since it is a single piston caliper. Shim the rotor as necessary for caliper position. Since it's "custom," you will have to figure that part out. While you are at it, be SURE the rim is centered in the fork. The left side axle spacer controls the wheel position.

Good luck, those things can be a can o' worms.
Jim

[ExplodingCoffinEmporium]

The appearance of this setup from the first picture just seems off, but it could be my eyes. It looks like the caliper and rotor are sitting cockeyed and pitched at an angle offset from the hub. Is that the case DrSweden?

If you want to spend a few bucks the mityvac works great for bleeding brakes and it’s varying application makes it well worth the price to me. I would second JB in saying a relaxed gravity bleed with everything opened up and topped off is always an easy way to avoid headaches. You could always bench bleed it as well to check for leaks or issues. Sometimes it’s easier off the bike I’ve found.

[docmel]

I like the compressor version of the mityvac It runs of an air compressor line. I have found the hand pump version just doesnt do the job half the time, its a PIA to hold the tip against the bleeder screw as well. Plus the compressor version will bleed a truck or car brake system in a flash' a MC super fast. I have pumped the hand version to death and it barely works

[JBinNC]

My Mighty Vac works really well. Too well as it turns out. I was bleeding a car with it and found that on the left front wheel I was pulling air past the master cylinder rear seal and down to the caliper. Realized I had to be a little more gentle with it.

Jim

[ExplodingCoffinEmporium]

My Mighty Vac works really well. Too well as it turns out. I was bleeding a car with it and found that on the left front wheel I was pulling air past the master cylinder rear seal and down to the caliper. Realized I had to be a little more gentle with it.

Jim

It’s funny really, I’ve heard such polarizing reviews over the years regarding the mityvac but never anyone in the middle. I’ve personally had similar experiences, works great with no issues and the one we have has been around the block for at least a decade and a half. I’ve also had bros that hate the damn thing and sent it to the trash heap shortly after getting it.

[DrSweden]

Tnx Jim, what is the issue with 5.1? I got the only issue is that you don't mix it with others, but it just have other properties for heat, wear, but not mechanical? The reason I went with 5.1 is that it doesn't react with air like the 5 does?

I will try to understand if wheel is centered and if caliper floats.

[confab]

The 35MM front end that came on my bike had brake calipers that looked identical to those.. And I had to rebuild them when I had this problem.

I don't know what was behind and around the brake pistons, but it had the consistency of oily grass and it had the pistons bound in place. It literally took a slide hammer to remove them, when you should be able to push them out of the bore with a little shop air.

I didn't have to change the pistons themselves and I forget what they were made of, but I know on cars they used to be chromed and corrosion under the plating will cause the pistons to bind also.

The good news is rebuilding the MC and wheel cylinder is cheap and easy to do.. It really shouldn't be that hard to bleed. You should have a working brake in a few minutes if everything is in good shape and any air left will kinda combine into larger air bubbles you can remove after it sits overnight.

I know some people will bleed them and then swear by locking the handle down overnight with a zip tie after bleeding to help get rid of the little "Soda Fizz" air that is left.

As was noted above, single sided calipers must be able to slide back and forth on their pins to work. If you have a bind there, you will burn pads and it will never work.

[confab]

PS: On one or two occasions on cars, I have seen a rubber line fail internally and act like a check valve between the MC and a wheel cylinder or caliper.

It is rare, but it is something nobody is looking for. I know I sure wasn't.

If you're taking it apart again, blowing through the hose should make it obvious if this is the case.