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Replace the master cylinder?

  • Thread starter Thread starter KRT
  • Start date Start date
K

KRT

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So background info: Bought my first bike (this gs750es) about two months ago and it's been needing some overdue TLC since. Master cylinder reservoir was painted shut and the fluid inside was black. The brake lever felt extremly stiff, then while you're still squeezing the lever, it would loosen up a little, but not much.

I didnt really know what I was doing when I bled the front brakes. I just let it all drain out of the left anti-drive nipple, but didn't refill the reservoir, so now the lever has no resistance (sidenote: I can still feel where the lever gets snagged, then loosens up), and the bike no front brakes. I tried refilling the reservoir, and followed what some other people posted in other threads about 'priming the master cylinder,' but no luck.

Do I have a giant air bubble, a busted master cylinder, or both?
 
Giant air bubble.
The very best way is to Vaccum bleed the system. (thats what I do)
Vaccum bleeder rig at Sears or wherever is about $40.00. Cheap in my opinion.
But...If you feel you must go the Macgyver route, here's how.
Fill up the master, then gravity bleed the calipers 'till you stop hearing air, and start seeing fluid. (keep it full) Once the calipers and lines have fluid in them, pull the master cylinder off and bench bleed it. Then reinstall it and bleed it again at it's brake hose fitting. You should have pressure then. One you have pressure, then bleed the calipers, and the anti dive again. (None of my bikes have the anti dive, so I'm sure someone has a procedure for that).

You'll get most of the air out, but I doubt you'll get it all. That brings us back to recommendation number one: Vaccum bleeding.

Btw, You'll get plenty of replies with all sorts of other crazy procedures...I tried every one thats been posted. Vaccum bleeding is the fastest, cleanest, and easiest way.
 
Last edited:
Vacuum bleed does work but I don't recommend it in this case. You should take the system apart and clean out the gunk.

One trick to get the air bubble out and prime the system is to get a piece of clear tubing and attached it to the brake bleeder nipple on the caliper. Crack open the nipple and suck on the hose - you should be able to draw fluid down out of the master and prime the piston- watch the fluid level as it goes up the tubing and don't go too high for obvious reasons. Also, don't let the master reservoir fluid get too low or you will just draw more air into the system. If you can not draw fluid down into the system this way, the bleed hole between the master cylinder resevoir and the piston is plugged - time to clean.

Good luck.
 
honestly if the fluid was that crappy inside and you are gonna have to rebleed anyhow just take everythng apart and clean it up really well. theres only one thing keeping you from the rear bumper in front of you, why rush it. imho

its really a pretty easy undertaking you just have to get your hands dirty and figure it out.

nick
 
Yup, never never fool around with brakes....your life depends on them don't ever forget.

If things are as dirty as you say , you need to clean everything, master cylinder, lines and calipers. While cleaning and rebuilding masters and calipers is very doeable when it comes to lines replace them. Stainless steel/kevlar lines are best. Long lasting and don't deform like rubber. Gives you much better feel at the lever.

For a first timer, this can be intimidating but it isn't a hard job, just time consuming and fiddly. If you don't feel up to it get a professional to do it.

If you want to tackle it get a manual for your bike and review the tutorial on BassCliff's page.

Cleaning the master cyclinder is quite straight forward but you may have issues with the calipers as sometimes it can be difficult to get the pistons out.

With systems that have been neglected there will be the possibility of rust and crystalized brake fluid in the calipers. Very common and sometimes a bad problem. The caliper bodies are aluminum and the pistons are hard chromed steel. The problem is that water does get into the system and rust forms in the caliper bores attacking the pistons. Pistons can become pitted and the rust can jam them in the bores. This all has to be cleaned up. A word of caution if the pistons are pitted ( in the area that sits in the bore) and the chrome has flaked off.....do not reuse...replace. Pitted pistons can re rust again quickly and this can lead to seizing in the bore....i.e. no brakes when you need 'em.

Another word of caution when cleaning brake systems only use the appropriate brake fluid. Never use spray cleaners or other petroleum products as they can leave residues that are not compatable with the brake fluid you put in and can cause other problems.

While we all generally would encourage you to do your own work, this is one area where you can't fool around. If you don't feel up to it please have someone who knows what they are doing sort them out for you.

Feel free to ask all the questions you want and we will get you up to speed in no time.

Good luck with the project and lets see some pics.

cheers,
Spyug
 
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