What causes poor brakes 66bonneville .shoes booster master are new
Has hard pedal seems to need a lot of presure to stop new booster master
has good vac. I remember these brakes only needed a lite touch to stop
There may be an adjustment for the booster but I can't tell you how. I would also bleed out the brake system at all four corners as your brake cylinders may be all gummed up.
a quick test for the booster system is when the car is not running, pump the pedal until hard, keep pressure on the pedal, start the car the pedal should drop down a bit this shows you have power boost assist. Did you check the valve that plugs into the booster, I have seen these go bad.
Check valve as mentioned above, still possible bad booster, and I assume you have the vacuum line connected to a good 17 in.lb. vacuum source? The motor itself draws good vacuum? New vacuum hose for the booster?
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