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Booster hissing inside cab-help please

4K views 8 replies 3 participants last post by  .Taco Lloyd 
#1 ·
I just changed all my vacuum lines, and now my brakes don't work, When the pedal is depresses its a constant steady release of air by the pedal. Like its a release valve or something. The motor starts to quit a little also. HELP PLRASE I need to drive to work tomorrow. How come this allways happens on Sunday.
 
#2 ·
My first guess is that you missed a connection, made at least one leaky connection, or scrambled connections when replacing the vacuum lines ... :shrug:
 
#3 ·
I was chasing a PO171 code- large vacuum leak, or fuel. . Replaced fuel pump and filter. Nothing !! Truck allways seemed starved for gas. Would not start hot. Replaced all vacuum hoses. Then the sound of the air leak. Another forum had a thread about PO170 vacuum leak thru booster, blown diaphragm. Plugged large hose no more CEL. No more brakes Ordered new booster, mail delivery only. My brakes have been spongy even after every.thing else was replaced. These trucks can be no fun when codes can mean so many things. I hate throwing money at symptoms. $ 300 to do it all myself, $1200 or more for a mechanic.
 
#4 ·
Have you not swapped out the brake booster yet?

One possible interpretation is that you've successfully isolated the booster as the main leak source.
 
#5 ·
The booster is ordered. I cant get one from ether major supplier. Vacuum air seems to go right thru this booster. When I push hard the motor shakes and wants to stall.I capped the booster line removed the cell , Never run better in 2 years. I have to try to drive without power brakes for a couple days. Ive found 4 different threads on this with a PO171 cel. I will reply when finished
 
#6 ·
I put the booster in, its quiet , stops better. Less spongy feel . It kills me though everything on my brakes has been replaced, fully bled. The peddle still goes low , wish it was at the top, is it an adjustment ???
 
#7 ·
Usually adjusting the rear brakes (clearance from shoes to drum) gets rid of a lot of the excess pedal travel. Also, the clearance between the booster pushrod and master cylinder should be zero. Use caution because over adjustment here can cause brakes to drag,overheat, then lock
 
#8 ·
Here (attached) are the procedures for checking brake pedal free play / travel and setting zero clearance for the brake booster push rod.
 

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