what are they?

my truck seems to have a hard time stopping occasionally. the pedal can get soft or hard sometimes too.

when it gets soft i can slam the pedal to the floor and the truck doesn't slow down.
when it gets really hard, it doesn't budge and the truck doesn't slow down.

i might have a vacuum leak as the motor idles high. would a vacuum leak affect the power brake booster? also does the stopping coincide with the tranny downshifting?

 

Sounds like you may have two problems. Try fixing the vacuum leak first so you know that you are getting plenty of boost. This should prevent the pedal from getting hard. Secondly, it is possible that the master cylinder is leaking internally, assuming that you have no external leaks in the system anywhere. I would do a thorough search of the brake lines and hoses front to rear. Brake lines love to rust and leak. Hoses could be breaking down internally causing intermittent problems too. Be sure to pull the drums and check the wheel cylinders for leaks too. Gently pull back the boots and look for wetness. Good luck!

 

what are known vacuum areas that are prone to go bad on these older trucks?