Just bought the truck the other day. Sometimes the brakes work good, sometimes the pedal is really hard and the truck doesn't want to stop fast at ALL. To the point where it feels dangerous to be driving it around, so I want to get this fixed as soon as possible (daily driver truck).

I also noticed that sometimes the "brake" dash light comes on too. I understand that this indicates low vacuum in the system, does this mean that it's likely the vacuum pump, or could it still be the brake booster?

If possible I'd like to only replace one of the two, I'm a little short on funds at the moment. I plan on trying to find a vacuum gauge to borrow at one of the auto parts shops here tomorrow, so that I can do some tests. I plan on testing, at idle and after stepping on the brakes a couple times, the vacuum level at the line that goes to the booster, as well as the main line that goes to the splitter. Anywhere else I should test?

Thanks,

Zac

 

Vacuum pump is the prime suspect. The sensor that turns on the light is between the vac pump and the booster, so if it goes on, it's more likely the pump.

 

Also check for vacuum leaks. If you're lucky, it could just be a loose hose somewhere.
It does sound like a bad vac pump though.

 

I agree ^ but would say the first thing to check is the alternator and vac pump for a loose belt. The vac pump being driven from the alternator pulley means that if *either* belt is loose the vac assist is intermittent, which trips the light and makes the brakes hard.

 

Good point! I completely forgot about that.

 

You know, actually my alternator belt (the main one not the one to the vac pump) is a little loose. I noticed this the other day. When running, I can see the part that goes down to the harmonic balance (I think that's what it is) wiggling back and forth a few inches or so. When I get my water pump installed and the belts back on there, I'll see if I can tighten it any. Would I just want to pull the alternator to the side as much as possible before tightening the bolt on the slider?

 

That belt always wiggles some, just because of how far it is between the harmonic balance and the alternator. Its a toss up between over-tightening it to get out the belt slap but risking the alternator bearing wearing out early. A few people have figured out a way to put an idler pulley in there to help keep the belt in line.

I can't remember for sure, but I believe you have to loosen the vac pump belt, then tighten the alternator belt, and finally tighten the vac pump belt. If you don't loosen the vac pump, you may not be able to properly tighten the alternator.

 

I got another question.. I'm thinking about just trying to find a vac pump off a truck at the pick and pull, would I need to find one off another 6.9L or are there any other engines that would fit, like the 7.3?

If prices are good for them, I might try and get a brake booster while I'm at it and hope that both of them are in working order.

 

vac pumps off a 7.3 with v-belts will work. The ones from a serpentine belt truck will not work, they mount differently.

Check the tightness of your belts, the condition of the vac hoses, and how much the vac pump sucks. Don't just randomly replace the pump until you test to see if that is the actual problem.