Cleaning Master Cylinder

I just took apart the front master cylinder from my '78 650 and man is it nasty in there! There is lots of rust coloured gunk, and after I took off the plastic reservoir there was a lot of sawdust looking stuff. No idea what it is.

Anyway, I was wondering what is safe to use to clean this out? Should I stick with just using brake fluid or can I use anything? Can I run the whole assembly under water? I know water is bad for brake fluid, but I won't be installing if for quite a while so it will be completely dry by then. I'm thinking there must be rubber parts in there so I don't want to use anything to damage them.

Any help would be appreciated.

if its that bad, you need to take it all apart and replace the rubber parts
you can use brake cleaner or carb cleaner and the piston bore may need some attention

Try to use brake fluid and a brush first, before any other chemicals. Some carb cleaners may hurt the rubber parts. I have used isopropyl alcohol to clean my master cylinder with no problem. I don't know if denatured alcohol will harm the rubber parts, so unless some one else knows stay away from denatured alcohol.

On a 78 650 (if you have the trapezoid shaped cylinder) then the rebuild parts are hard to find. Redline Cycle has a page of master cylinder rebuild kits, and one is listed for a 1000 A model. That is the one to use. I'll see if I can find the link.

Edit: I notice you have the dual disc so it's easier to get parts. Here is the link, compare your old parts to the pictures. You'll probably need the first or second kit shown.

tinyurl.com/36d9sd

Post edited by: OKC_Kent, at: 2007/12/18 13:24

Thanks guys, great info. I never had time to take it fully apart last night, hope to tonight so I can see what needs replacing. Thanks for the link OKC_Kent, never realized it would be that expensive for a couple of small parts

Thanks again.

When I went to change the brake fluid in my 81 650 this year, the master cylinder looked pretty funky (no sawdust looking stuff though), plus I had to drill out the screws for the cover, after rounding them out with an impact driver. I found another, cheap, on e-bay, and bought that before I screwed around with one on the bike.

The original had been working well, so I decided to see if it could be revived. Plus the replacement I got, needed to have the brake light switch configured differently. First I put a sheet of plastic over the tank, just in case of a spill. I used a 5mm tap to chase the threads in the original screw holes, I suctioned out the old fluid, blocked the holes draining to the piston at the bottom of the resivour, and then cleaned out the inside with paper towels and Q-tips. After the inside was cleaned well, I refilled it with fresh DOT 3 brake fluid. I put a clear hose to a container from the caliper bleed valve, opened it half a turn, and started pumping in the fresh fluid. The old stuff came out looking pretty nasty. After it finally came through clear for a while, I topped it back off, and it seems to be working pretty well. I'll have to get it on the road for a while to see if there is any fade, or if it won't hold pressure, but it seems pretty good for now. It's surprising how resiliant some of these old parts are.

Thanks MSGAmling. I have mine completely off the bike and disconnected from the caliper. I figure I should first clean out the sawdust stuff first. I'll then do as you mentioned and pump through some clean fluid to flush everything out. I'll also see at that point how well it's working. Hopefully it will be fine and I won't have to buy a rebuild kit.

Thanks.

I understand the not wanting to rebuild idea, but think about this, its your butt on the bike, and no brakes or not so good brakes, or even worse, brakes that fail when you need them,not a good idea.
I would rethink rebuilding the brakes, a few bucks now may save your butt later

BSKZ650 wrote:

a few bucks now may save your butt later

yea,your not in a cage,that thing you dent could be your head

Oh I totally agree with you. I'm not going to risk myself because of a few bucks. With that said, I'm not going to rebuild it either if it doesn't need attention. I finished cleaning it last night and visually everything looks to be ok. I'll get it put back on the bike and see how it operates.