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Discussion Starter · #1 ·
I was riding my 2005 vn750 home when I felt resistance against me then I noticed my front brake lever wouldn’t squeeze anymore it was stuck and even tho it wasn’t pulled in. my front tire was completely locked up had to leave in a parking lot got everything to clean master cylinder any idea what the problem is and if I need to clean the piston I can’t find a video of someone cleaning brake caliper for my bike
 

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You're gonna have to take some stuff apart. I would imagine either the piston is stuck or there is a collapsed brake line. The front braking system is pretty simple on these but does have quite a few hoses to inspect. You will likely benefit from a full teardown of the front brakes to inspect and make sure the hoses blow through easily and that the piston and master cylinder are operating properly.
 

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I would open the bleed screw on a caliper, the try to compress the caliper piston without using excess force. If you can't compress the caliper, then you know it's probably stuck.

If the fluid has been dirty there's a good chance the fluid return hole in the master cyl could be clogged.

 

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With no/low fluid in the master cylinder, (but enough in the system to work the brakes initially but not have the symptoms of air in the system) you may want to consider the master cylinder. While the calipers may need attention, I would look to the master cylinder first. Especially if you were ever super low on fluid for a while. If the level is really low moisture can get into the system since brake fluid absorbs it like crazy. That will increase the probability of problems caused by the rubber in the system deteriorating (there are a lot of rubber parts in the brake system). Also the contaminated fluid breaks down, mixes with the water (and rubber bits) resulting in mud like gunk that plugs up the small ports and causes problems with the calipers/pistons.

There 2 ports in the bottom of the master cylinder bowl. One port only circulates the fluid in the master cylinder and the other port only allows fluid into and out of the master cylinder. The one nearest the brake line running to the calipers is the pressure port. (If you take the hose loose from the maser cylinder and the calipers release, that will provide more information.) That port provides the brake fluid to the calipers when the lever is pulled.

The second “hole” is the (pressure) relief port. When you release the lever, the brake hoses are under pressure and the piston in the master cylinder needs to return to "center". There is a spring in the master cylinder to assist in the release of the pressure in the hoses and to return the piston to its center position. That port allows fluid to flow back and forth behind the piston of the master cylinder so the brake fluid pushed into the hose can return after actuating the calipers (thus equalizing the pressure in the master cylinder piston and allowing the calipers to retract).

If the relief port is plugged or blocked, or the spring in the piston is “stuck”, or for some other reason the piston will not retract to center, then the pressure in the system will not be released and the pads in the calipers will not retract away from the disc. If the pressure port is blocked then no fluid will be able to flow either in or out of the hoses Both ports need to be open for the system to function as designed.
 

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Discussion Starter · #5 ·
I’ve cleaned a master cyclometer before and know about the bleed screw I’m more worried that if it’s not the problem should I clean the break caliper pistons I’ve seen people pressurizing them with clamps should it work the same way on my bike ?
 

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Discussion Starter · #7 ·
This is what the master cylinder looks like I tried to take calipers off tonight but lost day light I think I’m just gonna clean the break callipers and pistons then flush the system
 

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Wow that is clean. :) Have you been able to get the caliper to release at all? If the piston will not retract sometimes it is necessary to help it. On early disc brake calipers you cloud use a 3 - 4 inch c clamp to press the piston back enough to fit new pads. Leaving the bleed screws open while you compress the piston will help since you do not have to push the fluid back into the master cylinder. Just be careful not to introduce air into the system. Always make sure there is fluid at the bleeder. I do not recall if the piston is such that it has to be rotated (usually backwards) to compress it enough to fit the pads. I don't think it is. The procedure should be in "the book."
 

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The compressed air is used to push the piston out of the caliper "sleeve" if you are going to completely take it apart. Do you have access to one?
 

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Discussion Starter · #10 ·
Ya I got c clamps to fit it and if the master cylinder looks good I have a good feeling that it’s the pistons the brake pads just stay clamped they’ve loosened up over th past couple days
 

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The c-clamp will not help remove the piston if you need to. It is for pushing the piston in so the pads will clear when the caliper is installed over the disc. However if they have loosened a bit that will help you take it apart and remove the calipers if you need to. It is still possible the master cylinder is not operating properly even though the pictures show the inside of the reservoir and the fluid look pretty good. The fluid level is low and close to allowing air into the system. Being that low moisture could condense inside the reservoir over time.

If the fluid became contaminated the piston in the MC may be stuck. Also the hoses could be damaged, either soft and vulnerable to swelling or collapsed inside. Lastly there is a greater chance of rust on the piston in the caliper or inside the caliper. Also the caliper seals may have gotten hard and allowed moisture in.

If it were my bike I would add more fluid to the master cylinder reservoir and then remove the lower ends of both hoses and place them into brake fluid. Then try and operate the brake lever. If it works properly the problem is likely in the caliper(s). If no fluid is pumped through the hoses the master cylinder is likely in need of service.

Pay attention to the appearance of the fluid coming out of the hoses to see how good it looks. If a hose has failed internally I would expect to see evidence of it as fluid is pumped through it. Being disconnected it should be able to move fluid through even a collapsed hose and a swollen/soft hose will also. Do not move the brake lever at all unless the hoses are in fluid and there is fluid in the reservoir while you are diagnosing the problem.
 

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Discussion Starter · #12 ·
Okay so remove the hose from each side of the break callipers put them in brake fluid if the lever pulls fine it’s probaly the calipers/pistons I saw someone online clean the pistons without taking them completely out he pressed the brake lever to push them about half way out used brake cleaner then used clamps to push them back in would this be a good way to approach it since it’s not currently in my garage
 

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Yes those are the hoses that need to be disconnected to perform the test Green1 is referring to.
I agree that since the brakes locked it’s probably an issue with the calipers (obstruction, sticking). My brakes went last year and the lines were the problem. The symptoms were different from what you’re seeing. I couldn’t get the system to pressurize, the lines were most likely degrading from the inside. I tried with no luck and took it to a mechanic who basically ended up rebuilding the MC and replaced the lines altogether.
 

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That would be the "standard" way of doing it. You will need to remove the caliper assemble from the bike to work on it most easily. It is really hard to say what will need to be done to take it apart. The piston If the seals are hard and shot I would plan on a full caliper rebuild. Thorn is correct that brake cleaner may not be good on the seals but if the calipers need rebuilt those will get replaced. However it may not help depending on way the pistons are seized. I use it to remove grime, dust and debris but it does not really get rid of rust. Also it evaporates fast and can be hard on other surfaces (paint and the like).

I can proceed in several ways in my shop but I don't know how you should. I have started using EvapoRust on a great deal of parts to free them up and remove rust.
It s non corrosive but you need to be able to complete the disassembly to inspect everything and make sure the piston is not badly pitted I see a kit on line that has new pistons along with the seals and assorted bits needed. But that kit is pretty expensive to get if you really don't need it. Over $50.00.

What have you done to date?
 

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That would be the "standard" way of doing it. You will need to remove the caliper assemble from the bike to work on it most easily. It is really hard to say what will need to be done to take it apart. The piston If the seals are hard and shot I would plan on a full caliper rebuild. Thorn is correct that brake cleaner may not be good on the seals but if the calipers need rebuilt those will get replaced. However it may not help depending on way the pistons are seized. I use it to remove grime, dust and debris but it does not really get rid of rust. Also it evaporates fast and can be hard on other surfaces (paint and the like).

I can proceed in several ways in my shop but I don't know how you should. I have started using EvapoRust on a great deal of parts to free them up and remove rust.
It s non corrosive but you need to be able to complete the disassembly to inspect everything and make sure the piston is not badly pitted I see a kit on line that has new pistons along with the seals and assorted bits needed. But that kit is pretty expensive to get if you really don't need it. Over $50.00.

What have you done to date?
Any updates on your brakes. I had to rebuild the calipers on my bike. The caliper piston had to be replaced on both .
 
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