sdseva

a buddy of mine has a 96 3500 dump bed. it all started with a hard brake pedal. now a new booster, master and vacuum pump and still no fixy! any help would be greatly appriciated as usual. thanks in advance everyone.
Seva

infidel

If this is happening at idle it's totally normal, the vacuum pump just can't keep up with repeated braking at low rpm.
If it's happening at higher rpm he has a vacuum leak or bad vacuum pump.

sdseva

he just put a new vac pump in with no change to the pedalall the vac lines look good so we are not really sure where else to look

mr bill

maybe a brake line failing internaly and not releasing

infidel

Easy way to check for a leak is to run the engine, shut down for five minutes then disconnect any vacuum hose.
If you don't hear a hiss there is a leak.
A leak free system will hold vacuum for hours if not days.

WestTN

I guess the bigger question would be, "Why is he pumping the brakes?"

BigIron70

Make sure the engine idle rpm is in spec. Must be 750 rpm in gear engine at operating temp 180 degrees with a/c on. If too low will not keep up vac supply when pumping brake pedal. Also check vac hoses to brake booster if they are soft or collasped replace them. Usually at vac pump connection the hose will get oil soaked and become soft. Make sure rear brakes are adjusted properly. Put a vac gauge into vac tree on firewall there should be a capped connection there. Start engine and let idle for a few minutes then turn off engine. Vac gauge should stay steady, if it drops off fast you have vac leak somewhere. Step on break pedal and vacumn gauge needle should drop slighly and then stop moving. If it continues to drop your booster has internal leakage. You should be able to make three brake application before vac is depleaded.

justme-

This is not normal- there is a problem. These systems are not known for awesome super vacuum or huge vac reserves, but pumping the pedal 3 times at idle was never enough to run my vac reserve out- constantly doing that when plowing snow. This year however it's a problem for mine so I know I either have a leak, bad booster, bad res, or bad pump. I'm going to rebuild the pump and start replacing all the vac lines.

Follow Infidel's advice to check the booster, then I would find and vac test the res. If good, I would start checking individual lines or just start replacing them.

sdseva

thanks guys, ill try a vac test(hiss test) tomorrow thats good idea.

Junk Man

Something else to check is to measure actual vacuum. Also make sure you don't have air in the brake lines and that your rear brakes are adjusted. Did you bench bleed the master?