What is the governing equation of vacuum brake booster based on (https://w​ww.mathwor​ks.com/hel​p/physmod/​hydro/exam​ples/vacuu​m-boosted-​tandem-mas​ter-cylind​er.html)?

Based on (https://www.mathworks.com/help/physmod/hydro/examples/vacuum-boosted-tandem-master-cylinder.html)

I want derive the equaiton of motion of vacuum brake, which uses to produce state equation. It consist of the all simscape element in this model

Please advise me