'81 GS550 Front Master Cylinder...not pumping

[wjwolfe82]

I've dis-assembled, cleaned & re-assembled several times and piston seems to move just fine but can't get it to pump fluid. I'm new to this so maybe just missing something. Thought maybe the seal on the piston is bad and I've searched for the replacement piston set (59600-33811) but it's not available anywhere...and neither a rebuild kit.
I removed and cleaned it because the brakes would gradually start locking up...as I rode, or just setting in the sun. I could crack the bleeder and it would release. Started with caliper repair but didn't help so moved on to the master. The reservoir was dirty...piston had some buildup but not that bad and I removed the reservoir before cleaning it all up.
But now I can't even get it to pump.
I'm probably gonna have to get an Ebay replacement but was hoping to keep it original as the rest of the bike is.
Any advice greatly appreciated.

master cylinder.jpg

[salty_monk]

There are rebuild kits out there.... Personally I would probably get a "stock look" from using one of these. Decent price & reputable source.

https://www.z1enterprises.com/search?p=1&q=brake%20master&product_list_order=name&product_list_dir=asc

[wjwolfe82]

Thanks for the link.
I've found rebuild kits for the rear but nothing shows up for the front for '81.

[93Bandit]

I recently did the same to one of my bikes. Disassembled and cleaned all parts, and reassembled with original seals. Normally it helps to "bench bleed" master cylinders to build pressure in the master cylinder before hooking up to the rest of the system. Before ordering anything, try this. I install the master cylinder on the handlebars like normal (or whatever position is easiest for you) but leave the brake line disconnected. Wrap rags around the handle bars and tank and anything else you don't want brake fluid on (all painted surfaces). I also put some cardboard on the floor below the master cylinder. Fill the reservoir with brake fluid. Then, slowly and carefully, I place my thumb tightly over the hole that the brake line banjo bolt threads into and pump the handle a few times, then carefully "burp" the hole with my thumb. Probably won't get much if any pressure the first several times, but eventually it'll begin to pick up fluid and build pressure. Do this procedure slowly because once it starts building pressure, it'll blow your thumb off the hole and spray brake fluid everywhere. Once it's building pressure, quickly bolt on the brake line. I usually have the caliper end of the brake line disconnected and hanging down over a small cup or something to catch fluid, then pump the handle to get fluid flowing through the line. Takes a little while but eventually it'll start dripping then streaming from the hose. Then I connect to the caliper and bleed all the air out of the system. This procedure has worked on the rear master cylinder for me as well.

I will probably get some comments regarding the potential mess this method creates, and yeah it'll ruin freshly painted items if you're not careful. There's probably a better way to do it, but this has always worked for me.

[Grimly]

The tiny little relief hole is blocked.
That's what caused the brakes to gradually lock up in the first place.
It's a common enough fault as master cylinders get old and the brake fluid hasn't (ever) been changed (enough).

[wjwolfe82]

I recently did the same to one of my bikes. Disassembled and cleaned all parts, and reassembled with original seals. Normally it helps to "bench bleed" master cylinders to build pressure in the master cylinder before hooking up to the rest of the system. Before ordering anything, try this. I install the master cylinder on the handlebars like normal (or whatever position is easiest for you) but leave the brake line disconnected. Wrap rags around the handle bars and tank and anything else you don't want brake fluid on (all painted surfaces). I also put some cardboard on the floor below the master cylinder. Fill the reservoir with brake fluid. Then, slowly and carefully, I place my thumb tightly over the hole that the brake line banjo bolt threads into and pump the handle a few times, then carefully "burp" the hole with my thumb. Probably won't get much if any pressure the first several times, but eventually it'll begin to pick up fluid and build pressure. Do this procedure slowly because once it starts building pressure, it'll blow your thumb off the hole and spray brake fluid everywhere. Once it's building pressure, quickly bolt on the brake line. I usually have the caliper end of the brake line disconnected and hanging down over a small cup or something to catch fluid, then pump the handle to get fluid flowing through the line. Takes a little while but eventually it'll start dripping then streaming from the hose. Then I connect to the caliper and bleed all the air out of the system. This procedure has worked on the rear master cylinder for me as well.

I will probably get some comments regarding the potential mess this method creates, and yeah it'll ruin freshly painted items if you're not careful. There's probably a better way to do it, but this has always worked for me.

By golly that worked!
It did take some time just like you said(and I almost gave up) but I eventually got pressure to build. It feels just a little spongy but it does engage the calipers...and I ran out of brake fluid but I think I can work it some more.
Thank you!!

[wjwolfe82]

The tiny little relief hole is blocked.
That's what caused the brakes to gradually lock up in the first place.
It's a common enough fault as master cylinders get old and the brake fluid hasn't (ever) been changed (enough).

Is the round type master different? There are two holes in the plastic reservoir but I only see the one hole about 1/8 inch diameter under it in the metal housing. There is a smaller indentation but it's metal...appears I would need to drill it out to pass through?

[Rich82GS750TZ]

Don't drill it. Try a very fine wire like high E guitar string or bristle from a wire brush.

[93Bandit]

By golly that worked!
It did take some time just like you said(and I almost gave up) but I eventually got pressure to build. It feels just a little spongy but it does engage the calipers...and I ran out of brake fluid but I think I can work it some more.
Thank you!!

Good to hear! I'm glad it worked for you.

[wjwolfe82]

Don't drill it. Try a very fine wire like high E guitar string or bristle from a wire brush.

Yeah, I've tried the wire brush bristle. It just seems to be a solid 'indentation'. And when I look down through the empty bore I only see evidence of the one larger hole. There are plenty of youtube videos and articles about cleaning a small relief hole but they are all about rectangular style masters.
So there still should be a tiny relief hole in these older round style masters?

[cowboyup3371]

Yes though to be honest it can be plugged. I had one that seemed like it wasn’t drilled from the factory and had to replace it. Work at it with brake cleaner and the piano wire as suggested but you may need to look for another.

[wjwolfe82]

Yes though to be honest it can be plugged. I had one that seemed like it wasn’t drilled from the factory and had to replace it. Work at it with brake cleaner and the piano wire as suggested but you may need to look for another.

I found another used round master on ebay and it is the same...just seems to be an 'indentation'...about 1 mm deep maybe. Maybe there is a smaller hole out the side of that indentation??
Anyhow I'm gonna keep the original one on it. I got it cleaned up nicely and it seems to be working well. We'll see once I get to ride it here in the next couple weeks if the gradual lockup goes away.
And...I spent way too much time getting that dang brake light switch back to working order...what a PITA. If I have to take that apart again I'm just gonna put on one of those banjo bolt switches.