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Discussion Starter · #1 ·
Hi all,

I have a GSX-R 750'91 and have some braking issues. The brakes do work correctly however very little breaking power. Any ideas on how to improve the breaking power? Thanks for the support
 

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Like the other folks have started asking, 'brakes not working well' is a huge can of worms. More info is needed! But also, brakes are a relatively simple system.

  • If you have 30 year old rubber brake lines, you should replace them regardless of having poor performance or not. High quality braided steel lines are a must, but won't solve any brake related issues unless your actual issue is a burst brake line.
  • Change the fluid, and do a proper bleed. Cheap and easy. You could be just compressing air bubbles in the line, or have super contaminated fluid. Worth changing just to make sure.
  • Pads can get contaminated, even modern good street pads. I accidentally used a rag I had washed in my washing machine to wipe down my rotors after scuffing them, and the oil residue from before the wash was still in the rag, and I put a thin coat of oil on the rotors. Pads soaked in that oil and they were ruined, even after cleaning them, cleaning the rotors, etc. Forks like to blow seals and then leak onto pads and rotors. Replace the pads if there is any chance they got oil or other stuff on them, or if it's been years.
  • Clean the rotors. Like I said, fork oil and grime like to get on the front rotors, and a quick scuff with light duty scotch brite and brake clean goes a long way. Just make sure to break in the pads again after doing the cleaning. If they've got a lot of vinyl record type groves, maybe time for a new set of rotors.
  • Check for fluid leaks. Use a ziptie to clamp the brake lever squeezed overnight. Not super hard, but relatively tight. If you come back the next morning and the lever stays clamped with no force, then you have a brake fluid leak somewhere or a worn master cylinder piston/seal if no fluid leaked.
  • Check for seized brake pistons. Especially on machines with 2, 4, 6, or 8 pistons up front in all the calipers, some can get stuck and ruin brake performance. If this is your problem, you just need to order the rebuild kit with all new seals and clean your calipers up and replace those rubber parts.
  • Not very likely but possible, your master cylinder may be worn or defective. They also sell rebuild kits for these. All you need to do is decide if rebuilding is worth it, or just get a new or upgrade master cyl.
  • Make sure you don't overfill your brake reservoir either. If there isn't enough air in the res, you can create a vacuum in there when pads start wearing, and the system won't work correctly.


If you do literally all of that and it still doesn't work, then something is put together wrong. I did all these steps when rebuilding my K4 750, and it has epic brake performance now, and when I got it, the brakes practically did not work at all. Hope this helps. Sorry for the ramble, but this is basically everything it could possibly be, and if your problem isn't on this list, you've got bigger problems.

-Mike
 

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Discussion Starter · #6 ·
Hi guys, first of all I would really like to thank you all for your time in taking some minutes to help. Attached I am sending some pics, looks pretty healthy to me, but I am not an expert. Also taking the opportunity to share a pic of my beatiful machine...
Tire Wheel Automotive tire Bicycle tire Motor vehicle

Wheel Tire Crankset Automotive tire Vehicle brake

Tire Wheel Automotive tire Hood Motor vehicle

Tire Wheel Fuel tank Automotive fuel system Automotive lighting

Land vehicle Vehicle Motor vehicle Automotive fuel system Automotive tire

Automotive tire Light Sports equipment Bicycle part Wheel

Automotive tire Automotive design Motor vehicle Automotive wheel system Gas

Sports equipment Sports gear Bicycles--Equipment and supplies Outdoor shoe Grey

Automotive tire Hood Motor vehicle Automotive lighting Bumper
 

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Discussion Starter · #7 ·
Like the other folks have started asking, 'brakes not working well' is a huge can of worms. More info is needed! But also, brakes are a relatively simple system.

  • If you have 30 year old rubber brake lines, you should replace them regardless of having poor performance or not. High quality braided steel lines are a must, but won't solve any brake related issues unless your actual issue is a burst brake line.
  • Change the fluid, and do a proper bleed. Cheap and easy. You could be just compressing air bubbles in the line, or have super contaminated fluid. Worth changing just to make sure.
  • Pads can get contaminated, even modern good street pads. I accidentally used a rag I had washed in my washing machine to wipe down my rotors after scuffing them, and the oil residue from before the wash was still in the rag, and I put a thin coat of oil on the rotors. Pads soaked in that oil and they were ruined, even after cleaning them, cleaning the rotors, etc. Forks like to blow seals and then leak onto pads and rotors. Replace the pads if there is any chance they got oil or other stuff on them, or if it's been years.
  • Clean the rotors. Like I said, fork oil and grime like to get on the front rotors, and a quick scuff with light duty scotch brite and brake clean goes a long way. Just make sure to break in the pads again after doing the cleaning. If they've got a lot of vinyl record type groves, maybe time for a new set of rotors.
  • Check for fluid leaks. Use a ziptie to clamp the brake lever squeezed overnight. Not super hard, but relatively tight. If you come back the next morning and the lever stays clamped with no force, then you have a brake fluid leak somewhere or a worn master cylinder piston/seal if no fluid leaked.
  • Check for seized brake pistons. Especially on machines with 2, 4, 6, or 8 pistons up front in all the calipers, some can get stuck and ruin brake performance. If this is your problem, you just need to order the rebuild kit with all new seals and clean your calipers up and replace those rubber parts.
  • Not very likely but possible, your master cylinder may be worn or defective. They also sell rebuild kits for these. All you need to do is decide if rebuilding is worth it, or just get a new or upgrade master cyl.
  • Make sure you don't overfill your brake reservoir either. If there isn't enough air in the res, you can create a vacuum in there when pads start wearing, and the system won't work correctly.


If you do literally all of that and it still doesn't work, then something is put together wrong. I did all these steps when rebuilding my K4 750, and it has epic brake performance now, and when I got it, the brakes practically did not work at all. Hope this helps. Sorry for the ramble, but this is basically everything it could possibly be, and if your problem isn't on this list, you've got bigger problems.

-Mike
I will definitely go through all these points...much thanks!
 

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Very nice machine you got there! Yeah it looks like it has seen some use, but actually looking right at it I don't visually see anything wrong. But of course the problems are not visible until you get inside some of the parts.

In case you haven't, get a shop manual for the bike, and it will show you how to address all of those things I mentioned.

On a side note, I don't know how the bike rides since it's obviously yours, but bikes this old with non-radially mounted calipers, non-drilled rotors, and I have no idea what type of pad is in there, those parts may just not brake as hard as the modern bikes. So depending on how poor the performance is, it may just be designed that way. I've ridden a few Harleys and other bikes, and sometimes they just have bad brakes.

If you can still slam on the brakes, lock the rear up on purpose, and stop hard enough that you think you could possibly either lose traction on the front tire or stoppie, then the system is probably working as designed, even if you have to squeeze the heck out of the lever. My ninja was that way with stock brakes. I thought they weren't that bad of brakes until I hit the track and got upgrade brake parts, and then if I ride another of the same bike with stock brakes, it feels literally scary how bad they are.

So take my recommendations with a grain of salt. If you can't stop the bike hard, then something is wrong. If it's just poor performance, could just be made that way and just be old. Decide for yourself what it is, hope this helps.

-Mike
 

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Sparky gave you a great list to go through. Also, in one of the pics, the rotor looks pretty worn/rough. Have you mic'd them for thickness? And if they are grooved badly, they won't work well at all ...
 

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Sparky gave you a great list to go through. Also, in one of the pics, the rotor looks pretty worn/rough. Have you mic'd them for thickness? And if they are grooved badly, they won't work well at all ...
True, especially if you just put on new pads and expect the pad to wear into all the groves during break in, lol. Good catch on the thickness check too, worth the two seconds to measure.

-Mike
 

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Discussion Starter · #11 ·
thanks guys for all the comments. I changed the break pads, cleaned the rotors and break callipers. Cleaned the rotor with break cleaning liquid however ... the rotors don't move at all. I have tried to loosen the rivets that grab the rotors using screw and additional bolt but with not much success. Shouldn't their be some king of movement of the rotor? I believe this is what could be causes the vibrations. The rotors are to dam "fixed" to the bottom part of the rotor. being flouting disks, they should move, even if only very little...right? I now have much better breaking power but vibrations....could it be from the rotors.

Tire Wheel Automotive tire Tread Motor vehicle
 

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Discussion Starter · #12 ·
Sparky gave you a great list to go through. Also, in one of the pics, the rotor looks pretty worn/rough. Have you mic'd them for thickness? And if they are grooved badly, they won't work well at all ...
For thickness they seem OK. Hugely improved the breaking power but now I have vibrations. These rotors should move...right? They are stiff as hell...
 

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They usually move, but I think I have seen a few examples where they work without literally jiggling around. Sometimes they are hard to move by hand if they are a bit tighter. With the bike in neutral, saddle up on it with no stands, and hold the front brake, then with your legs and arms, bounce the front forks. If you hear a knock when you pull and push the forks, that is the sound of the rotors shifting on the buttons. I wouldn't worry if they aren't shifting, but if they are warped, that is bad.

If the warping/vibration only became present after the pad change, something went together incorrectly when you changed the pads. Did you push all the brake pistons back into the calipers before the change? Did they all move with the same amount of effort?

-Mike
 

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Discussion Starter · #15 ·
They usually move, but I think I have seen a few examples where they work without literally jiggling around. Sometimes they are hard to move by hand if they are a bit tighter. With the bike in neutral, saddle up on it with no stands, and hold the front brake, then with your legs and arms, bounce the front forks. If you hear a knock when you pull and push the forks, that is the sound of the rotors shifting on the buttons. I wouldn't worry if they aren't shifting, but if they are warped, that is bad.

If the warping/vibration only became present after the pad change, something went together incorrectly when you changed the pads. Did you push all the brake pistons back into the calipers before the change? Did they all move with the same amount of effort?

-Mike
Hi Mike, I did push the pistons in before installing the new pads. However when pressing the break leaver to clean the pistons only one came out (the closest to the oil), when I pulled manually back in, the opposite one came out. The other two did not move at all. The vibration/warping already existed previous to the clear and pad change.
 

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Discussion Starter · #16 ·
Hi Mike, I did push the pistons in before installing the new pads. However when pressing the break leaver to clean the pistons only one came out (the closest to the oil), when I pulled manually back in, the opposite one came out. The other two did not move at all. The vibration/warping already existed previous to the clear and pad change.
BTW - I also bled the breaks to ensure no air in the system
 

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Vibration (brake chatter) will prevent full braking capacity as the vibrations are essentially pushing the pistons back in limiting full pressure.
Depending on the disk the outer steel rotor may not move on the aluminum carrier, but the bobbins (spools) should spin by hand in place. you can clean them out by bolting through them with some washers, then putting the bolt in a electric drill. spin it until it moves freely with some brake cleaner.

the disks may be warped which means they may need to be replaced, or if thick enough, they can be reground and reused. You can check the disk for run-out with a dial indicator, if you have a mag base, slap it on the fork tube and spin the wheel in your stand. max runnout is usually around 0.30mm (0.012") but if its close to that i would double check the spec for your bike. Its hard to tell but the disks look like they may be glazed and this can be a sign of warp-age.

Another likely cause for vibration in braking can be traced back to improper wheel mounting. after the front axle is torqued down the pinch bolts should be loosened and the suspension should be cycled several times to settle the forks on the axle. then torque the pinch bolts. This is a common problem.
 
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