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Discussion Starter #1
I had the caliper rebuilt by a shop recently and the front brake was working fine until yesterday when it seized up again. Shop says the master cylinder needs to be rebuilt now so I bought a rebuild kit and plan on doing it myself.

I've seen a few YouTube videos on other bikes and it seems pretty straight-forward but I was wondering if anyone had any other tips or resources they use to rebuild one for a '91 Intruder 750, thanks all.
 

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Master cylinders are generally very easy to rebuild. Remove the lever and the dust boot. Sometimes there is a spacer, sometimes the lever pushes right on the piston. Look in the bore and you'll most likely see a snap ring. Remove that and the piston will come out, install the new one, re-assemble.

Now when you're talking a bike that's pushing 30 years old corrosion can be a major issue especially if brake fluid was neglected for decades as it often is. Once you get it apart inspect the inside of the piston bore for pitting/corrosion. Light amounts can be cleaned up by just shoving a bit of scotch-brite pad in there and spinning it around, but if the new piston still does not slide in smoothly/freely, you probably should replace the M/C.
 

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Discussion Starter #3
Thanks for the tips.


The front brake seems to work fine when left alone to cool down. After riding it for a few minutes and getting it warmed up it will seize on me again, any ideas??
 

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Thanks for the tips.


The front brake seems to work fine when left alone to cool down. After riding it for a few minutes and getting it warmed up it will seize on me again, any ideas??
Hi Kenny. Welcome to the VR from snowy Alberta Canada. Yes, I’m looking out the front window and watching the snow fall.

As to your hydraulic brake problem, the fact that the lockup occurs after the brakes warm up indicates a strong likelihood that expanding brake fluid is doing the lock up for you.

The fluid will naturally expand when ir warms but there is a feature in the master cylinder that is supposed to allow the expanded fluid back to the cylinder so there is something that is preventing that from happening.

1. In the bottom of the master cylinder reservoir are two holes. One large and one small.

The large hole is behind the piston to refill the pumping chamber in front of the piston cup when you pump the lever. (You can ask how that happens later). The small hole is in front of the piston - on the caliper side- to allow the fluid to return to the reservoir when you release the brake lever and also as the fluid expands.

2. The second possible cause occurs inside the brake hose when the hose deteriorates due to age of the system. The lining of the hose acts as a one way valve to hold the fluid in the caliper.

I would check the master cylinder reservoir first.

Two things are the major causes of the problem in this location.

1. Debris in the bottom of the reservoir that gets stuck in the little hole - and by little, I’m talking 0.010” -

2. The mispositioning of the piston in its resting position. When the brake is released the piston Normally sits between the two holes. If the piston is forced to sit slightly in front of the hole, fluid can’t return.

a. Corrosion in the cylinder or dirt behind the piston.

b. The push rod between the lever and the piston is not in its proper place due to a reassembly problem at some time in the past.

Hope this gives you some results because this is all that comes to mind.
 

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Discussion Starter #5
Hi Kenny. Welcome to the VR from snowy Alberta Canada. Yes, I’m looking out the front window and watching the snow fall.

As to your hydraulic brake problem, the fact that the lockup occurs after the brakes warm up indicates a strong likelihood that expanding brake fluid is doing the lock up for you.

The fluid will naturally expand when ir warms but there is a feature in the master cylinder that is supposed to allow the expanded fluid back to the cylinder so there is something that is preventing that from happening.

1. In the bottom of the master cylinder reservoir are two holes. One large and one small.

The large hole is behind the piston to refill the pumping chamber in front of the piston cup when you pump the lever. (You can ask how that happens later). The small hole is in front of the piston - on the caliper side- to allow the fluid to return to the reservoir when you release the brake lever and also as the fluid expands.

2. The second possible cause occurs inside the brake hose when the hose deteriorates due to age of the system. The lining of the hose acts as a one way valve to hold the fluid in the caliper.

I would check the master cylinder reservoir first.

Two things are the major causes of the problem in this location.

1. Debris in the bottom of the reservoir that gets stuck in the little hole - and by little, I’m talking 0.010” -

2. The mispositioning of the piston in its resting position. When the brake is released the piston Normally sits between the two holes. If the piston is forced to sit slightly in front of the hole, fluid can’t return.

a. Corrosion in the cylinder or dirt behind the piston.

b. The push rod between the lever and the piston is not in its proper place due to a reassembly problem at some time in the past.

Hope this gives you some results because this is all that comes to mind.

Hello from Florida,


What's this "snow" you speak of?? Lol.


Thanks for the insight. Yesterday I removed the MC cap and slowly squeezed the brake lever, no fluid squirted up from the MC at all, does this mean I have debris in the holes? The fluid looked good (was bled about a month ago) and I could not see anything clogging the holes.


It's having the same issue, caliper stays clamped and won't release until several hours later. I have a MC rebuild kit on order but I'm hoping it's a simpler fix.
 

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Hello from Florida,


What's this "snow" you speak of?? Lol.


Thanks for the insight. Yesterday I removed the MC cap and slowly squeezed the brake lever, no fluid squirted up from the MC at all, does this mean I have debris in the holes? The fluid looked good (was bled about a month ago) and I could not see anything clogging the holes.


It's having the same issue, caliper stays clamped and won't release until several hours later. I have a MC rebuild kit on order but I'm hoping it's a simpler fix.
Yup. Got about 6” of the whit stuff. Good thing is it will melt away over the next week. Bad thing is the grain harvest is less than half dome around here. SIL stil has over 2000 acres to do.


There should at least be a swirl in the reservoir when you pull the lever. - if not an out right spirt of fluid flying up.

Something is blocking that little port.
 

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Discussion Starter #7
There should at least be a swirl in the reservoir when you pull the lever. - if not an out right spirt of fluid flying up.

Something is blocking that little port.

I only see one hole and I used a small pointy object to clean it out but there was nothing clogging it. Where is the other hole located? Is it under a flap?


 

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The large hole is the one for refilling the pumping chamber when pumping the brakes especially when bleeding air from the system or if there is excessive pad/shoe travel and adjustment is needed.

It looks like the “return hole” has a valve of some sort -that is not a usual design. There could be a problem wit that valv/return.

If you get is so it is rideable again and the brake seizes up again just loosen the hose fitting on the master cylinder. If the brake releases, the problem is in the m/cylinder.

If it doesn’t release then, loosen the caliper fitting. That will tell you about the hos.

No release with caliper hose loosened, means something wron with the caliper.
 

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Discussion Starter #9
Just rebuilt the MC, brand new brake line, and fresh dot3.

Only issue now is I can’t seem to bleed the brakes. Opened the mc cap, put a clear tube on the caliper nipple, several squeezes of the lever, hold, loosen nipple and allow fluid and air to escape. Repeated this process several times and not much air escaped and the lever still has no resistance.

I even bled the mc by removing the banjo bolt and placing my finger over the hold and pumped the lever.

Any ideas?
 

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Just rebuilt the MC, brand new brake line, and fresh dot3.

Only issue now is I can’t seem to bleed the brakes. Opened the mc cap, put a clear tube on the caliper nipple, several squeezes of the lever, hold, loosen nipple and allow fluid and air to escape. Repeated this process several times and not much air escaped and the lever still has no resistance.

I even bled the mc by removing the banjo bolt and placing my finger over the hold and pumped the lever.

Any ideas?
After rebuilding the MC it is much easier to bleed the rest of the brakes if you bleed the MC before installing it on the handlebars. That way you make sure that the cylinder is level and no air is trapped in a high spot.

You can also reverse bleed by forcing brake fluid in through the bleed screw and out the cylinder reservoir. In that case make sure that the cylinder is as level as possible.

Bleeding these brakes can be a bit frustrating at times.
 

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Brake fluid is very effective at removing paint! Be certain to drape the areas where fluid could get onto and manage what you are doing. I don't know if plastic is affected by brake fluid or not but wouldn't chance it. Locktite is also very bad to get on plastic if you didn't know.
 

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Discussion Starter #12
I'm at my wits end here.

Even with the caliper nipple closed and full brake fluid in the mc I cannot build up any pressure. When trying to bleed the system I did notice some dark swirls starting to appear in the mc (did not see any bubbles or squirting of brake fluid when pumping the lever though).

I can see a small amount of fluid/air coming out of the caliper nipple hose but nothing significant.
 

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I'm at my wits end here.

Even with the caliper nipple closed and full brake fluid in the mc I cannot build up any pressure. When trying to bleed the system I did notice some dark swirls starting to appear in the mc (did not see any bubbles or squirting of brake fluid when pumping the lever though).

I can see a small amount of fluid/air coming out of the caliper nipple hose but nothing significant.

Have you bled the Master cylinder with it removed from the handlebars and mounted horizontally in a vise?

The Situation sounds like there is air in the pumping chamber in the master cylinder.
 

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I've run into this problem with other bikes. Usually its because I've reversed the rubber sleeve during reassembly and it won't work that way no matter what you do. I would disassemble the master cylinder and make sure its assembled correctly.
 

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Discussion Starter #15
https://i.imgur.com/vYiZAM2.jpg

Tried reverse bleeding today and found some interesting info. The mc is actually blocking fluid from filling the reservoir (I unscrewed the brake line and was able to reverse bleed it right to the union bolt).

So I took my mc apart again, do the cups look right? One doesn’t...
 

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https://i.imgur.com/vYiZAM2.jpg

Tried reverse bleeding today and found some interesting info. The mc is actually blocking fluid from filling the reservoir (I unscrewed the brake line and was able to reverse bleed it right to the union bolt).

So I took my mc apart again, do the cups look right? One doesn’t...

The seals on the piston looks correct.

Did you disassemble the return valve circled in red?

That is where the fluid returns through to the reservoir when reverse bleeding or brake is released.
 

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Discussion Starter #17
The seals on the piston looks correct.

Did you disassemble the return valve circled in red?

That is where the fluid returns through to the reservoir when reverse bleeding or brake is released.

Thanks for verifying the piston placement.


The return valve did not have anything to disassemble, the only thing I did was poke through the 2 holes to make sure nothing was blocking them.


Sorry for the late replies, was traveling for work for the past 2 weeks so the bike just sat there but now I'm ready to tackle this issue again.


To recap: caliper was rebuilt and new pads installed by a shop. I rebuilt the master cylinder using a kit, replaced the brake line, and tried bleeding the fluid.


I can bleed the mc separately, I can bleed via the caliper only if I unbolt the brake line at the mc. Otherwise, once everything in buttoned back up I can no longer bleed the system (usual way nor reverse bleed).


The issue seems to stem from the mc but I have no clue what else I can do now.
 

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Discussion Starter #18
So I just wanted to thank everyone that provided some input for this problem. I ended up buying a master cylinder off a '95 and after installing it I was able to bleed the brakes like normal. So far the bike has been stopping fine, haven't tried taking out on the road yet but it did well around my complex. Thanks again and be safe, all.
 
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