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Master cylinder disassembly/rebuild

danleym

Just got it firing!
GT Contributor
Joined
Jul 14, 2022
Messages
15
Location
USA
Well, having removed the throttle (my last post...), I got the master cylinder off, and if I fill the reservoir and squeeze the lever, nothing happens. No fluid moves through it. Which would explain the difficulty in bleeding the brakes I've been experiencing. I read through the shop manual I found on here, and the step to disassemble the master cylinder references a specialty tool used to press the internals out. Anyone ever torn theirs down before? Have any tips for getting the inside bits to make their way outside?
 
Try this, fill the master cylinder, squeeze the lever, then put your thumb over the outlet of the master cylinder and release the lever, do this a couple of times. The vacuum caused by releasing the lever will hopefully draw fluid into the internals. Also make sure the piston returns fully to its end stop as there is a small hole it has to uncover.
 
Be advised It may be cost effective to just replace the master cylinder. Contact Todd via the contact tab in the store. He can supply new Brembo brake components at a good price. Also Brembo has made rebuild kits unavailable in the US thanks to our lawyers.
 
I tried putting my thumb over the end- I could feel pressure and suction as I pumped the lever, but no fluid made it through. I'll try adjusting the stop before I attempt disassembly any more.

John- I found a rebuild kit elsewhere for $23, compared to a master cylinder for $160 at the same place. I haven't shopped that around at all- the prices may well be better here, I'll get it touch with Todd and see! I only started down the disassembly route after finding that other kit.

I really think it just needs a good cleaning. I failed to change the fluid frequently enough, and it got pretty murky and nasty in there. Might still work well if I can just get it apart and clean it, then reassemble with the same components. On the other hand, for $23 I might as well replace some seals while I'm in there.
 
The V65 is an old bike. Master cylinders don't last forever and being able to stop is a good thing. For the safe of safety, it would be worth a new master.
 
Wow $23! Is it in stock?
What bike is this and what's the kit number?
I just ordered a kit for my V65GT for €50 from Europe and 8 weeks lead time.
 
I have no experience with Moto Guzzi brakes, but remember the experience I had with a 1982 Yamaha many years ago. Braking effectiveness and feel deteriorated as pads continued to wear. I tried rebuilding the master cylinder and then I could not bleed the brakes at all. I eventually discovered the small hole that allows fluid to flow from the reservoir into the cylinder/piston chamber was plugged by corrosion. That made it impossible to bleed the brakes and was keeping fluid from pushing the caliper pistons out to compensate for pad wear. I was able to clean that small hole by using a sewing needle as a drill and then bleed the brakes with fresh fluid. I had no more brake issues as long as I had that bike.
 
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I haven't had a chance to dig into it further- busy day.

John, I hear you on the safety thing. It's an old bike. A new master is probably the safest choice. But I think a master cylinder is a simple enough mechanism, that either I can get it working or I can't, if I can't I'll buy a new one, if I can I have little expectation that it will suddenly fail after being made to work correctly. I feel comfortable enough in my mechanical abilities to recognize one that's right and one that isn't. And I like fiddling. So I think I'm going to keep fiddling with this a bit longer before I throw in the towel.

My bike is an 84 V65 SP.

The rebuild kit is a Brembo part, number 17659352. Listed for 75-84 Guzzis, it does not include the V65 GT as one of the applications. It's on MG Cycle. No idea what international shipping is like for them, but it says in stock.
 
Thanks for the info.
I agree Stein is top notch and they answer calls/emails.

I am getting parts now to do the same rebuild on both brakes this winter.
Mine is slightly different, but your post may help me figure out how it's done.
I'll keep posting as I work through it then. Might be a few days before I get to it again, but I'll update when I do.
 
On my big block Tonti I once found the tiny hole that feeds fluid to the piston chamber blocked. It was difficult to see.
Even my vacuum bleeder wouldn't draw fluid through.
 
I think this is likely the problem. Still haven't had a chance to mess with it, but I think there's a good chance my lever wasn't adjusted right, and it wasn't clearing the hole. I can look in it and see the two holes, and they don't appear to be blocked. I was vacuum bleeding, too, and getting nothing at all. After years of bleeding brakes on cars and trying all different methods, I definitely prefer the vacuum bleeder.
 
I think this is likely the problem. Still haven't had a chance to mess with it, but I think there's a good chance my lever wasn't adjusted right, and it wasn't clearing the hole. I can look in it and see the two holes, and they don't appear to be blocked. I was vacuum bleeding, too, and getting nothing at all. After years of bleeding brakes on cars and trying all different methods, I definitely prefer the vacuum bleeder.
I've found reverse fill works the best. Fill the master cylinder from the caliper. That pushes air up and out.
 
I've found reverse fill works the best. Fill the master cylinder from the caliper. That pushes air up and out.
Tried that one too. Pushed a little fluid in, got real hard to depress the plunger, then brake fluid leaked everywhere. Definitely have a blockage somewhere. I put new brake lines on, and when i had the calipers apart I ran some so brake clean through them, and that went fine, so I'm fairly confident it's in the master cylinder. I am looking forward to trying the reverse fill though, I'd never heard of it before I started digging into this but I'm going to give it a go.

Been a busy few days, but now that I have my deck rebuilt and the family canoe repainted, I might be able to justify a little time for a more selfish project...
 
Put it all back together, fiddled with the stop on the lever, and all of a sudden it works! Haven't had time to fully bleed the system, but there's definitely fluid getting past the master now. Glad to know that's all it was. Anyone have input on proper setting of the lever stop? I pretty much backed the screw all the way out, and it's working there, but there must be some happy medium?
 
Bled it today, nice firm handle, at least while sitting still. Vacuum bleeder FTW. Just need to get the rest of the bike back together and road test!
 
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