How does the glide ratio change with velocity in a glider with air brakes deployed?
This can be easily answered with a spreadsheet. Start with speed $v$ going from 20 m/s to 50 m/s. Add a column for the zero-lift drag coefficient and keep in mind that the increasing Reynolds number will change that value over speed, like that: $$c_{D0}=c_{D0_{Ref.}}\cdot\left(\frac{v_{Ref.}}{v}\right)^{-0.3}$$
I picked as reference values $c_{D0_{Ref.}}$ = 0.01 and $v_{Ref.}$ = 25 m/s. Adjust as required.
Next, add a column for the zero-lift drag coefficient with speed brakes deployed and add a constant value. I picked 0.015 for the example plot below.

Now we need to add induced drag and start with a new column for the lift coefficient $c_L$. Since weight does not change with speed brake setting, one will do for both configurations:$$c_L=\frac{2\cdot m\cdot g}{\rho\cdot v^2\cdot S}$$
I used 350 kg for the mass $m$, 1.225 for air density $\rho$ and 10 m² for wing area $S$. The induced drag coefficient $c_{Di}$ will be $$c_{Di}=\frac{c_L^2}{\pi\cdot AR\cdot\epsilon}$$
My sample glider has an aspect ratio $AR$ of 20 and an Oswald factor $\epsilon$ of 0.98. Again, adjust as required. For extra correctness you might want to add an extra column for the real-life lift coefficient with beginning stall. Keep the "clean" lift coefficient for induced drag but plot all drag coefficients over that extra column where the lift coefficient grows less than the linear value once it is above the threshold where flow separation starts on the wing, say 1.25. Now your speed will not be correct for lift coefficients above that threshold, but your induced drag will become very realistic.

Now add another column for induced drag with speed brakes deployed. In oder to model the distorted lift distribution over span with speed brakes, reduce the Oswald factor to maybe 0.7. Add both drag coefficients for the total drag coefficient: $$c_D=c_{D0}+c{Di}$$

The result should look something like that. The index wF is for the configuration with speed brakes deployed:



The glide ratio E is the ratio $\frac{c_L}{c_D}$ and plotted on the right Y axis. Note that the best glide is cut almost in half and moves from $c_L$ = 0.7 to about $c_L$ = 1.0 for speed brakes deployed. I did not add the extra drag at high lift, so your result might look slightly different.