Engine/Transmission (1994 - 1998) Hooked up vacuum gauge, drops to zero upon shutdown

[KRStone]

I hooked up a vacuum gauge on my '96.

gauge climbed steadily at idle, then higher upon throttle.

Drops significantly upon depressing brake pedal, 5 pedal pushes and it's near zero.



Upon shutdown it drops to zero immediately.

Did brake booster check after shutdown, it does hold some vacuum for a few presses of the pedal.



I've suspected a vacuum leak for some time.

I looked at the hoses, but nothing obvious.



I need to troubleshoot it, but I think it's time (17 year old vehicle) to convert to truck air hose. I just can't find any threads where someone has done this replacement/upgrade.



Thoughts?

KRS

 

[GAmes]

I just can't find any threads where someone has done this replacement/upgrade.

You could be the first! Take some pictures and post the results. I sure would be interested.

 

[ata]

I think you are going to be chasing a GHOST. There is no vacuum reserve tank on your truck except in the booster brake. When you shut down the motor the vacuum pump stops. The vacuum drops to zero but the brake booster will hold vacuum for 1 to 3 steps and then it is gone. There is no need for vacuum when the truck is sitting (motor not running) except for the brakes by FED. Law.

 

[Missouri Mule]

I have to agree with ata. You have two check valves one at the booster and one at the pump (they keep oil from entering the system).

 

[JKosten]

What lead you to do this test?

THe most common place to have leaks in the vacuum system are the HVAC controls or the 4WD CAD control system.

 

[GAmes]

I thought the most common were the T junction above the fuel filter that inadvertantly gets pulled apart and the vacuum line to the cruise control servo under the battery.

 

[KRStone]

Doing this test because i have lost vacuum brake assist when repeatedly braking while offroad at slow speeds.

Cruise doesnt work.

Pacbrake not working, however i think i need a new can for it.

 

[moparguy]

KR you may have a problem,,,,or not. I first noticed my 95 loosing braking at low speed while maneuvering a long horse trailer is a tight spot on a hill back in 96 or 97. Caused a moment of high anxiety till I got it all sorted out. The dealer and my research verified it was just the nature of the beast. I raised idle speed a couple hundred rpm and things were much better. Idle around 1,000 seems to work best (although I like it better at the 700 mark).

One solution some used was to install a vacuum tank, search may yield something.

Added in edit; https://www.turbodieselregister.com...ss-of-vaccum-for-brakes&highlight=vacuum+tank

 

[KRStone]

Replaced brake booster, vacuum holding well and braking improved.

Placed a handheld vacuum pump on the removed brake booster, it never built uo vacuum!

 

[BD-Hector]

way back when there was a recall for the vacuum supply issue, also check the vacuum cylinder on the exhaust brake to make sure its not leaking making it look like the pump /booster.