Sporster | Bleeding brakes

published 23/03/2016 | last modified 23/03/2016

What happened ?

Maintenance gets interesting (and bitchy) when a problem arises you cannot solve in the first place. You have to find what’s wrong. This is what happened to me recently and I take pleasure in telling you the whole story.

STEP 1 : I decide to change from DOT 4 to DOT 5

Why ? Because DOT 5 meets the requirements of the bike, according to the User Manuel. DOT 4 does not. What’s more, DOT 4 is supposed to be aggressive, when DOT 5 is not. Another reason might be I wanted to have a go at it :)

Bleeding the brake, so far everything goes according to plan

STEP 2 : Pressure is not building up

After bleeding the brake and getting rid of bubbles, I start pumping on the brake to gain pressure. I get absolutely none. Hence no brake.

No f**** brake, no matter what :(

STEP 3 : Master-cylinder stripping down

I have no leak I could see and/or hear. I think 3 options are possible : master-cylinder, durit, caliper.

Most logical is master-cylinder, because this is where pressure builds up. What’s more, having no pressure at all does not look like a bubbles problem (with bubbles you have little pressure). Having no pressure at all looks like I screwed up my gaskets.

Starting to unscrew to access the master-cylinder
God I love my bike but what on Earth is this ? A tiny pin I must not loose nor damage.
Secondary gasket on cylinder looks ok.
Hu Ho primary gasket is damaged. No wonder I had no pressure building up.

So I think “Well, I’m done here. Let’s put a new one back on ride”.

When put back on, I still had no pression.

Darn !

So I remove the caliper and try to put it upwards, to remove some hypothetical bubbles. Nothing happens.

Then I unscrew the master-cylinder to see wether my gaskets have been damaged or not put correctly. They are fine.

Hence I go back to DOT 4 and I start gaining pressure, but it fades away once the caliper has braken once. I can brake, but only once, then I have to pump to get pressure again.

STEP 4 : Findind what’s wrong

Sometimes you have to step back and think. What is my breaking system composed of ?

  1. 1 master-cylinder (looks all right since I have spent 8 hours or so on it)
  2. 1 durit (looks all right)
  3. 1 sensor for the rear braking light (should be ok)
  4. 1 durit (looks all right)
  5. 1 caliper (not investigated yet)

I think “I am gonna have to take the calliper off (and remove the rear wheel again in the process – yeah) and dissassemble it : check the gaskets, see what I can find.”

Then I realize that a caliper is also made of a purging screw. I wonder “What if the problem comes from that screw ?”.

Is that screw airtight when put in place ? Well, no. I can vacuum little bits of brake fluid when it is tight.

You can see that little dark circle down into the hole : I screwed too tight, hence I damaged the caliper, hence the air leak.

STEP 5 : back to normal

Very luckily, the thread for the purging screw and the one for the durit (incoming flow of braking fluid), are the same. Then I could switch. Otherwise I would have tried to put a little gasket in the hole but I can not say for sure I would have worked.

Durit and purging screw are inverted. That caliper is pretty basic hence this is possible.

My suggestions when bleedind your brakes

  1. be very, very gentle with the purging screw. Do it by hand when purging. Then, once it is done, add an extra 1/8 turn (NO MORE unless otherwise specified in your manual) with a socket

  2. be very gentle when pumping on the brake to regain pressure, especially when the DOT has just been replaced : you are putting pressure on small gaskets who are not lubrified yet
  3. do analyse what you see : no pressure at all -> gaskets ; pressure then no pressure, then pressure -> tiny leak ; pressure but your brake is not firm enough -> air bubbles
  4. probably the one and only tool you need : a good and flexible piece of silicon durit ; I bought items that are useless now (on this bike anyway)
  5. your bike runs ok on DOT 4 but the manual says you should use DOT 5 ? Stick to DOT 4. Same applies with DOT 5.
  6. otherwise the DOT you should use is written on the plug of your braking fluid reservoir
  7. do not trust people who tell you to wait 24 hours for the DOT to settle and the bubbles to disappear ; this is bullshit ; an hydraulic system works, or does not (see photo below)
Friend told me to wait for 12 hours untils the bike sorts the problem out by itself ! I thought I should put candles to help the magic.