brake issues

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What is the best way to test the power brake booster to see if it is working?

get in the car , with the engine off , pump the brake pedal  5 or 6 times   , then with your foot still hard on the brake , start it up ! if booster is working correctly  brake pedal should sink a little 1/4 to 1/2 inch may be little bit more ,( its not set in stone ( lot of variables ) , but you should feel the pedal sink a little on start up ,

also nothing to do with the brake booster now , ive just learnt something new in this thread    
  I always thought only drum brake cars need a residual pressure valve , due to the drum springs retracting the wheel cylinder pistons , soon as brake pedal is released , & next time you press the brakes with out the valve the pedal will be momentarily spongy for one pumpish.  & discs never needed a residual valve only a brake proportioning valve