GSXS750 Brake Upgrade

GSX-S750 Brake Upgrade

GSX-S750 Brake Upgrade

 

The original GSR750 had a huge upgrade into the more popular GSX-S750 in 2017, there were upgrades in styling, power, suspension and most significantly  with the brakes.

The original GSR750 had 2 Twin piston sliding Tokico calipers and the GSX-S750 had twin radial 4 piston Nissin Calipers, however the Master cylinder is still a 14mm axial master cylinder although Nissin not Tokico.

Thus a huge increase in caliper piston area but no change in master cylinder area.

Stock Brakes

GSR 750 

GSR750 Brakes

  • Twin Piston Calipers
  • Master cylinder 14mm diameter, area 153mm square
  • 2* Caliper, 2*27mm diameter, area 2290mm square
  • Ratio  1:15

GSX-S 750 

GSXS750 Brakes

  • Radial 4 Piston calipers
  • Master cylinder 14mm diameter, area 153mm square
  • 2* Caliper, 2*30mm diameter, 2* 34mm diameter,  area 3230mm square
  • Ratio 1:21

 

Thus the GSX-S750 has  3 times the caliper piston area of the GSR 750 but the same master cylinder so the master cylinder to piston area ratio goes from 1:15, to 1:21, only counting the pistons on one side of the caliper (although the GSXS has pistons on both sides they travel half the distance)

 

Upgraded Brakes

The GSX-S1000 comes with a Nissin 19mm Radial Master cylinder and this can be swopped over to the GSXS750 giving a better ratio

GSX-S 750 with GSXS1000 master cylinder

  • Master cylinder 19mm diameter, area 284mm square
  • 2* Caliper, 2*30mm diameter, 2* 34mm diameter,  area 3230mm square
  • Ratio 1:12.5

Thus a much better ratio as well as the advantages of a radial master cylinder

Radial vs Axial

In an axial master setup, the master cylinder’s bore runs perpendicular to the lever travel. A protrusion on the lever pushes the plunger in the cylinder to force fluid to the calipers, which squeeze the pads and make you stop. Axial master cylinders are still in use on all but the highest-performance bikes. How come? They work well and have been in use for decades. Why reinvent the wheel?

GSX-S Brake Upgrade Axial vs Radial Master Cylinder

Radial cylinders move the piston in the master cylinder in a direction parallel to the lever travel. That’s it. That’s the big difference.

So in practical terms, what does a radial master cylinder get us? Quite simply, improved feel through parts with more rigidity. Rigidity is key to obtaining consistent, crisp braking feel. Many riders say a radial master gives more precise feel through the feedback the rider gets.

GSX-S750 Brake Upgrade

Because of the inbalance of the master cylinder to caliper on the GSX-S750 this causes a very long lever travel with a very soft feel, I prefer a firmer feel with less travel particularly when the brakes get hot and the travel increases further.

I decided to swap the 750 Master cylinder (Nissin 14mm axial) with the master cylinder from the GSXS1000 (Nissin 19mm Radial)

GSXS Brake Upgrade

Nissin 19mm Radial Master cylinder
GSXS1000 Radial Master Cylinder 19mm
GSXS750 Axial Master Cylinder 14mm
GSXS750 Axial Master Cylinder 14mm

 

The 1000 master cylinder also has the advantage of a bleed nipple and a slightly larger brake fluid reservoir.

However the fluid outlet is underneath the master cylinder rather than on the end, this meant purchasing a new brake hose from the master cylinder to the main line connection as it needs to be about 3 cm longer. (At some stage the other hoses will be upgraded to Stainless Steel Braided Hoses too)

master cylinder brake hose

Reverse Bleeding

Bleeding the brakes proved to be very difficult, Using conventional bleeding, or vacuum bleeding  (with priming the master cylinder first) both proving to be useless and not able to get any pressure in the system.  Ultimately reverse bleeding worked first time.

I attached a flexible clear pipe to the left front caliper bleed nipple with the other end to a 50ml syringe, I then filled the syringe and pipe with fresh brake fluid, holding it up to clear any bubbles.  Opened the bleed nipple and squeezed the syringe to fill the brake system from the caliper end, this worked perfectly

 

GSXS750 Reverse Bleeding

Results

The result is a much firmer, more confidence inspiring brake lever, and the 750 now has superb brakes to go with the increase in performance from the decat and ECU Flash whilst still looking stock.

 

 

 

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