GSXR 750 brake problems

[Rizzla11]

Hi all,

I have a 1986 GSXR750 running what I believe is a 1992 front wheel and discs. I've had these wheels on here for a while and have always had problems getting the brakes setup correctly. I'm running standard callipers with 1 mm offset spacer between the calliper and fork leg to centre them, however getting them to operate without binding has been a bit of a nightmare. I've checked the spacing across the front axle which is correct for the setup I'm running (standard 1986 speedo drive with a spacer to align) and a correct spacer on the non-speedo side.

I've also rebuilt the callipers with new pistons and seals, cleaned everything before setting up and bleeding the system again. The master cylinder is working correctly and don't believe I have an issue there. One possible problem could be the original callipers from the 1986 model were design to operate with a 4.5mm width disc, whereas the later models ran a 5mm width disc. The original 1986 callipers also ran a thicker pad at 8.7mm, whereas the later model ran a 8.5mm pads. I'm thinking the square seal in the calliper isn't getting enough piston travel distance to deform correctly and recess the piston. Any thoughts? The discs also look a bit worn so thinking about what to do next, maybe replace like for like discs and run the later callipers with an adapter?

Cheers,

 

Al

[TonyGee]

have they got the anti-squeal shims in ?   ive had the same problem a while back and removing the shims fixed it.  

[Rizzla11]

1 hour ago, TonyGee said:

have they got the anti-squeal shims in ?   ive had the same problem a while back and removing the shims fixed it.  

No anti-squeal shims install, I just use a small amount of red grease or copper slip on the back plate.

[coombehouse]

I have had issues like this in the past caused by poor quality pads with the steel backplates not being flat. 

Failing that, have you checked that the fluid return hole in the reservoir is not blocked or obstructed.

[Rizzla11]

15 hours ago, coombehouse said:

I have had issues like this in the past caused by poor quality pads with the steel backplates not being flat. 

Failing that, have you checked that the fluid return hole in the reservoir is not blocked or obstructed.

Had another look today at the master cylinder and can see fluid returning to the reservoir as expected. Also bled the system again which gave a better response at the lever. I ended up taking approx 0.2mm brake material of each pad to allow the pistons to compress and recess correctly. I fairly sure the issue is later wheels with later thicker discs. I took out for a run today and not seeing any signs of brake binding. I think the limited throw in the pistons with wider discs isn't allowing the square seal or return to work correctly. I think I'll have to investigate running the original 86 discs on the later wheels to avoid this issue in future.

 

[dupersunc]

92 discs are larger diameter than 86 discs. You must have some sort of caliper adaptor for that combination to fit. Any pictures.

[Rizzla11]

56 minutes ago, dupersunc said:

92 discs are larger diameter than 86 discs. You must have some sort of caliper adaptor for that combination to fit. Any pictures.

I'll take some pictures. The discs clear the callipers ok, although they are fairly tight. Do you know the diameter of the 86 discs versus the 92?

[dupersunc]

92 are 310mmiirc 86 are around 280mm it's a big difference.

[Rizzla11]

On 11/14/2022 at 9:28 PM, dupersunc said:

92 are 310mmiirc 86 are around 280mm it's a big difference.

I founds some sizes on the web:

FRONT WHEELS:
Model Wheel Size Axle Size Disc Outside Diameter
1985-86 750 18x2.5" 15mm 11.8" (300mm)
1986 1100 18x2.75" 15mm 12.2" (310mm)
1987 750 18x2.5" 17mm 11.8" (300mm)
1987 1100 18x2.75" 17mm 12.2" (310mm)
1988 1100 18x2.75" (3 spoke type) 20mm 12.2" (310mm)
1988-89 750 17x3.5" 20mm 12.2" (310mm)
1989-90 1100 17x3.5" 20mm 12.2" (310mm)
1990-91 750 (U.S. '92 same) 17x3.5" 20mm 12.2" (310mm)

[TLRS]

Could there be any corrosion in the seal seats?

If you put the front up on a stand, how far can you spin the wheel?