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Brake booster dilemma

MEMO43

MEMO43

1/2 ton status
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Mar 24, 2018
Posts
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Location
Gardenrville NV
OK subject is an 83 k5 with crate sbc 350 (35,000miles), 700r4 with np208.
I replaced the booster about 3 months ago. Then noticed a hissing when applying the brakes, as well as shaking while at a stop.
Replaced all vacuum hoses.
Hose to booster
Pcv hose
Heater hoses as well as booster check valve, pcv. Still I get hissing when stepping on the brakes. I'm getting about 15" of vacuum through edelborock 1406 manifold port side at 750 rpms. Timing is at 17 initial 34 total.
I live at 5500 feet but work at 6500 feet.
Daily driver.
I'm on the 3rd booster all from Napa
When I spray carb cleaner around the intake, or any possible vac leak locations I get no change in idle speed.
The base of the carb I get an increase in idle.
Now I know I need to replace the gasket there.
What else can cause my booster to leak air when applying brakes. The sound can be heard underneath the dash when..
I'm almost at a lose here
Any help would be appreciated.
I'm a 911 operator and retired Marine, my next day off will be this mon (memorial day) an Tuesday after that it will be the following weekend.
Where should I start other than the carb gasket


Memo.
 
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it is bad internally when you are applying the brake the diaphragm moves there is either a small tare in it or a seal one of the rods is leaking. Replace the booster, hopefully is still under warranty.
 
Wes this is the 3rd booster
I changed two of them this weekend.
I'm at a total lose on this one. Never had to change so many "new" boosters all with the same end result
 
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if you only hear the leak in the cab when applying the brakes it's the booster. Only other solution is adjust to a rich mixture and turn the radio up. A richer the mixture will help with rough idle.

Idk maybe something in the brake travel, rod length master cylinder rod adjustment is causing the booster to be damaged
 
Thanks for the info
I'll play with the A/F mixture.
I'll check the travel rod as well
Hopefully something simple im just overlooking
 
sounds like the problem i had years ago .

83k10 truck removed cruise control and didnt know to remove the kill switch on the brake pedal under the dash that let vac from the engine open and dump the diaphragm for the cruise can . it would sit there and hissssssssss until brakes released .
 
Sweetk30 thanks for the info
I did notice I have two switches on the pedal
I assumed on was for the brake lights and the other for the 700r4.
I'll take a look and see which goes were.
I do have a cruise control on the turn signal lever. But since I've owned it its never had Cruise control, wonder if its another PO mystery.
Tucker that is a fancy booster from wildwood, but i already replaces the master cylinder..
hopefully I don't have to go through 7 booster before I find one that works.
On my next days off I'm going to either make a smoke machine an see if i have any other leaks or if its just a bad batch of boosters that are at the parts store
 
fire up the truck / stick head under dash and take hand and push pedal / if hisssss check the vac switch with a 1/4" or 5/16" black rubber vac hose to it .

in your clean up under the hood with all new hoses you might have re-hooked up a old hose that was pulled or replaced a hose they shoved something in to block it and you didnt know it .
 
Will do thanks
I'll even see about snapping a few pics of what I do or did'tt do
Hopefully it helps someone else, if anything to remind me of why I don't like new trucks.
 
There is a vacuum switch for the cruise control on the brake pedal. Unclip it and pull it off the bracket then operate the brakes, essentially you are eliminating that it's the switch for the cruise.

I've had poor luck with brake boosters also.
 
Ok will do
I can hear a click sound when I press the pedal
I wonder if this switch will also cause any vacuum leaks.
Tomorrow I plan on replaceing every vacumm line to see if that helps..
As well as try and make my own version of a smoke machine to find any other leaks
 
Good catch @sweetk30. I removed my cruise so long ago I forgot about the brake vacuum switch.
If the 700r4 is factory there should be a brake vacuum switch that unlocks the torque converter. Don't disconnect that.
 
i swapped 2 reman and a used booster before i found mine . . . that was almost 20 years ago now .

glad to have helped out :waytogo:
 
Were would the factory brake vacuum switch be?
Same place as the brake light switch.
The 700r4 is factory
The only thing I've done since owning is do a motor swap, as well as remove all the emissions stuff, but thay was done last year an havent had an issue since then, up until recently
 
Yes the two brake switches will be right next to each other inline with the brake pedal arm/bracket under dash
 
well thanks for all the help and pointers, ended up being the cruise control switch as mentioned by a few.
crawled under the dash with the engine running an was able to locate the vacuum hose with all its glory.
disconnected the hose from the manifold .15 cent rubber cap and now no more hissing.
Good thing I don't have cruise control other than my right foot :)
now to fine tune the timing and see if I can get my vacuum, higher than 15 inches. spoke with an edelbrock tech and gave me some pointers.
but that's for another post.
Once again thanks all for the replies.
 
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