How to figure out if lack of brakes is vacuum pump or booster?

shakazulu12 · Oct 1, 2013

Just had a lovely wakeup call in the dually when I suddenly had no boost on my brakes coming off the highway heavily loaded. Anyhow, pedal is firm as a rock, I'm just not getting any boost or assistance from somewhere. Oddly I don't have the ABS light flashing at all.

Anyhow, I gather I can check the vacuum with a gauge, except I don't know where it is. Anyone have a pic or diagram for me?

Other comments are appreciated as well if I'm missing something obvious. I am no amazing mechanic, but I can usually fix just about anything with a little direction.

Lethalthreat7.3 · Oct 1, 2013

There is a hose going to the booster. Verify that the vacuum pump is working. Disconnect the line and see if you have vacuum from the pump.
Boosters that have a failed diaphragm can be heard (sounds like an air leak) from the booster when the brake pedal is depressed.

shakazulu12 · Oct 1, 2013

I do have a small amount of vacuum from the pump. I just pulled the hose out of the booster and stuck my finger on the end. It's not much, but I can feel the vacuum. When the engine is shut off, I can hear an air leak sound like you are describing. I'm the only one home today so I don't have someone else to push the pedal while I'm under the hood. The air leak sound seems to be coming from the passenger side (hard to tell, it's stormy out right now).

shakazulu12 · Oct 1, 2013

All right, took the gauge out and yeah, it's only measuring 10 inches. Well below the factory rating of 18.

zilla68 · Oct 1, 2013

when you get the part, you will need a puller also

old man dave · Oct 1, 2013

Also make sure you have the vacuum pump mounting bolts back in place on the pump BEFORE you reinstall the pulley, otherwise they will not go back in with the pulley in place.