livewire

I have been trouble shooting a braking problem on my 96 4x4 auto. I have vacuum in the system but I takes a miniute to build up after starting. I have a vacuum gauage inside the cab. At idle the gauge reads 25 in.Hg. When I release the brake pedal it drops to 10 in.Hg or less. This causes the trouble light to come on. It will go back up but it takes about 10 seconds. The brake pedal will push to the floor. I have taken all the vacuum lines apart and tested them one at a time. The only one that is strange is the hose going to the brake booster. If I apply vacuum to the booster It will read 10 in.Hg and stay there, if I pump the vacuum pump more it will not go up it will stay at 10 in.Hg. Do you guys think this is normal? All the rest of the would build up more vacuum and hold it.
Thanks for any info or suggestions. John McHorney

tappet

You should have about 15 hg at idle it sounds lik you have done all of the checks i have not looked at mine but when you do the vacum check pull the hose not from the booster but from the engine and do the check there on some veicles there is a check valve if you are getting no air flow replace the check valve air must flow away from the booster replace if it flows in both directions also.. i just got into diesels so i hope the system is the same i think it is.
depress the pedel severl times to deplete any vacum remaining in the booster now depress the pedel and start the engine if the pedel goes down slightly the operation is normal. then release the pedel and let the engine idle for a few min. turn off the engine and depress the pedel several time slowley if the pedel goes down farther the first time but gradually rises after the second or third depression the booster is air tight. start the engine and depress the brake pedel then stop the engine with the pedel still depressed if there is no change in the reserve distance for 30 seconds the booster is air tight if the pedel feels hard when the engine is running the booster is bad

infidel

Totally normal for the vacuum to be slow to recover after braking at idle, the pump just can't keep up.
Dodge's solution is to add a vacuum reservoir, costs over $100, but you can come up with one on your own for less than $25.

Vacuum should read the same no matter where you measure in the system.

capt.Ron

I went through the exact same problem last week. have you tried switching your AC vent switch while watching the gauge with the engine running. Mine would run 27-28 as long as I didn't have the **** turned to recirculate or off. either of those positions gave me a 24-25. Now the brake was a completely different beast.
I tested the vacuum by pumping and releasing the pedal 3 times. The pressure would drop a little when I first got on the brake then drop alot momentarily when I got off of the brake. I waited 1-2 sec between releasing and getting back on the brakes.
below are the results Starting at 28

brake- 26
release-16
brake- 14
release-10
brake- 7
release- 3
After about 15-20 seconds the pressure will rise back up to 28 again.
my truck was giving me the brake and abs light when using the brakes as well as causing my vents to switch to defrost due to the lack of vacumm.
I decided to change the booster and all is well now. I can pump the brakes like a jack rabbit I have non of the problems that I was havng before changing the booster.
Now the first replacement booster had a bad control valve and was actually allowing the pedal to fall off of the brake light switch and apply light pressure to the brakes at the same time. I had to contact a company to find out that the bad control valve could cause that.
below is a link to the thread that I started when I was having the brake problem.
vacumm leak