MK3 rear brake bleed

I am having a bear of a time bleeding the rear brake on a MK3. The master cylinder is new. The flexible line is new. The caliper is rebuilt. New pads. It takes a lot of pedal movement to make it work. Is it possible that the brake switch is contributing to the problem? I know the rear brake is marginal but at least I would like minimal pedal travel.

seattle##gs said:

It takes a lot of pedal movement to make it work. Is it possible that the brake switch is contributing to the problem? I know the rear brake is marginal but at least I would like minimal pedal travel.

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As the caliper has been rebuilt and new pads fitted then it will take a while for the brake to bed in, but more likely there is some air still in the system.

Try removing the calliper from the bike and elevating it to above the master cylinder. space out the pads with a suitable spanner or similar. orientate the calliper to get the nipple at the highest point then bleed it. If this doesn't work, try moving the calliper to lower than the master cylinder with the nipple at the lowest point and using a syringe and suitable plastic tube on the nipple. pump fluid in from the nipple end. You need at least three hands and it will likely get messy. Consider removing the pads to avoid contaminating them with fluid and spacing the pistons out with something bigger. Use tywraps to hold the spacer in place.

If you pushed the inside caliper piston up against the bore, my experience is that the caliper won't bleed. Before I attach the brake line I shoot a dose of compressed air into the caliper. Such a method will also tell you if the caliper is (still) sticking. With air pressure the rear wheel should be locked tight, when you remove the are pressure the wheel should spin easily.

Bleeding is straight forward, unless you are one of the people that feel that the master needs to be pumped vigorously several times before opening the bleeder valve, this method rarely works. Instead, get an assistant and do the following GENTLE procedure: Have the assistant push the pedal down, and KEEP it there, while you open the bleeder valve, then shut the valve and have the assistant allow the pedal to return, then repeat. This way fluid moves through the system only one way. Keep the reservoir from going dry, and it helps to have a clear line plugged onto the bleeder valve and then put into a container containing brake fluid so the line exit is NOT allowed to be above the fluid level.

A Mity-Vac has worked for me, but I never found it to be nearly as reliable as what I wrote above. Filling the system by "injecting" brake fluid into the bleeder valve is another possibility, again I haven't seen this work as reliably as the hand method above.

Raising or lowering the caliper isn't necessary as the bleeder is high enough. It takes me about 3-5 minutes, with an assistant, to bleed a MK3 rear caliper.

I noticed once when I left the caliper (front wheel) dangling with no pucks and worked the master cylinder the pistons almost met before I discovered the error. I pushed them back into the caliper and, lo and behold ,I had significantly better lever position. This was a +++. I couldn't say why it worked. Any theories?
I have always bled the brakes using your above method, usually with complete success. But the rear is fighting me foe some reason.

Another quick way that builds on most of the above. The caliper was designed for the pre-MK3 fitment and as such has the fill hole and the combined vent line in the vertical position when fitted to the RHS slider. With the MK3 it seems to be a compromise and thus the fill and vent hole are slightly off vertical, but it is easy to get the air pocket out.

Bleed as much air as you can, you should have some feel in the lever, with the bleed screw shut off remove the bleed line, take the bike off the centre stand and lean to one side and just push it rock it back and forth a couple of times, lean bike other way and repeat, this allows the air trapped in the very top of the caliper to travel up to the bleed screw. Fit pipe and bleed, you will notice that a bit more air comes out and the lever feel will improve. 10 mins and the job is done.

Mk 111 Rear is easy. It's all about opening and closing the bleed valve at the right moments.

A Gunsons easy bleed kit or a large horse syringe fed through the bleed nipple will solve your problem, but you might have to make your own pipe to pipe connector ;

http://www.halfords.com/workshop-tools/tools/hand-tools/gunson-eezibleed