master cylinder help please

[swingermike]

Hi

My Honda silverwing recently failed it's MOT due to a binding front caliper. I cleaned the caliper enough to pass the MOT but it is still sticking and making for crap mpg. My brother in law suggested i checked the fluid in the front master cylinder resevoir to check there was a bubble popping up each time the brake lever was released which i did and no bubble. He says that this suggests gunge in the little hole in the resevoir and cleaning it with a pin should solve the proble. Unfortunately it hasn't so could anybody on here confirm that the absence of a bubble is indeed indicative of a problem (there is a bubble in the rear brake resevoir) and how i should best clear any gunk i.e drain fluid, use a syringe, use a longer probe then a pin).

Alternatively is stripping the master cylinder a job for an enthuisasticamateur armed with a workshop manual.

Cheers Mike

[BFG]

If the brake is binding I would put money on the caliper, not the master cylinder. Can you force the pistons back away from the disk (using an appropriate tool)?

The master cyl return can often get blocked, but this deserves greater examination before disassembly.

You should see a flow of fluid returning, which disturbs the surface of the fluid in the reservoir so I can see how this could be called a bubble. But an actual bubble means air and air is bad.

[Kickstart]

Hi

Not sure what you mean by bubble? If you mean that taking the cap off the reservior and pulling / releasing the brake you will see an air bubble rise from the hole in the bottom of the reservior then that is not true. You might see an air bubble come up but only if there is air in the system.

I would suggest either the caliper needs stripping and cleaning (take your time and be careful with the seals), it is sticking on the sliding pins on the caliper or the brake fluid reserviour is far too full.

All the best

Keith
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[swingermike]

sorry, poor choice of words. It isn't an bubble i'm looking for but a disturbance in the fluid to show that fluid is returning up the system.

i am reasonably confident that the pistons and seals are ok as i cleaned them until i could see the pistons were backing off when the lever was released and before the second MOT a mechanic cleaned them again to ensure they passed, although i'm pretty sure he didn't take the caliper off the bike or fit new seals.

Although it did pass it's MOT it didn't solve the problem entirely, just allowed the front wheel to spin enough after the front brake was applied to pass the MOT.

I'm not sure how to check the sliding pins on the caliper, they were cleaned and i applied a brake paste to them so i am assuming they are ok. i was able to make the caliper slide manually by pushing the two sides together when the caliper was off the disk.

If the above suggests it could be a blockage in the master cylinder any suggestions on the best way to clear it.

Thanks for the help, it is much appreciated.

Mike

[BFG]

You can get a master cylinder repair kit for not many pounds. Strip the master cyl, blow out any passages with compressed air. Pop in the new piston (or swap the seals if no piston provided), flush the whole system through with fresh brake fluid and bleed.

Maybe then do the same to the caliper when you find it didn't cure it.

[Kickstart]

Hi

You should get fluid coming back up into the reserviour, but if the piston is sticking then it will not move back and will not push fluid back.

The caliper should slide on the pins fairly easily.

New caliper seals are expensive for what they are but still not much for a bike with a limited number of brake caliper pistons. Generally there are 2 seals on each piston, the inner one actually sealing the fluid while the outer one just keeps dirt and muck out. Often the caliper will work fine for a short while with the outer seal missing but it will rapidly jam up. My guess is that this is missing / damaged (and there is probably a build up of corrosion in the grooves the seals sit in).

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[swingermike]

Thanks again.

i feel reasonably confident i can do a decent job of cleaning the calipers up and fitting new seals so i'll do that before farting about with the master cylinder.

I can't do until Friday so wait for my desperate posts then.

cheers Mike