failed booster question?
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Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 117
From: starship enterprise
failed booster question?
I do believe I know the answer already, but after my recent brake upgrades, I was looking over my truck, idling in the yard, stepping on the brake pedal occasionally, holding it depressed for extended periods so that I could do leak checks, and so on. The brakes have always felt a little bit soft, not like a concern but just I have to push the pedal. I blamed bigger tires until recently I installed normal tires, still no improvement. Well, I noticed today that when pedal was depressed, idle changed, went slightly not as smooth, and air was being drawn into the booster via the hole in the cab where the rod inputs. I am pretty sure this is a failed booster. That explains the "soft" brakes. Any other input? I was ready for the f350 tandem diaphragm upgrade anyway, so I will go on board and see the difference. No other brake system I ever had did this, so I feel it to be wrong.
Post Fiend
Joined: Dec 2006
Posts: 9,111
Likes: 3
From: Lexington Mo
To me that soft brake pedal sounds more like air in the brake system, or maybe the master cylinder itself. Possibly the rear brake shoes aren't adjusted out enough. I go through that quite often on my 79, Ka'Bluey.
With the engine not running, press the brake pedal until it's rock hard. Keep pressure on the brake pedal and start the engine. The brake pedal should go down. If it does not, then you have a vacuum booster problem.
Let us know what you find!
With the engine not running, press the brake pedal until it's rock hard. Keep pressure on the brake pedal and start the engine. The brake pedal should go down. If it does not, then you have a vacuum booster problem.
Let us know what you find!
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Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 117
From: starship enterprise
It passes all of the tests, as far as starting and pedal going down, but that continuous leak in the input rod seems odd and the engine idle change symptom is directly pointing at a booster issue or "vacuum leak" as the book calls it. Thanks. I might have a mc issue, but adjustment is not it, if anything they are a little tight, they make a little heat, the air could be, but I bled and bled, and haven't seen air for a couple times, but air can be hard to get all of it out.
Fleet Owner
Joined: Dec 2009
Posts: 20,678
Likes: 82
From: NorCal
When my booster failed, it basically started to lock up the front brakes after first brake application. Only after shutting off the engine and letting it sit for 15 minutes would the brake pressure bleed off enough to move again.... then the cycle would repeat.
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Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 117
From: starship enterprise
mine makes vacuum, and seals, then once you step on the pedal it is like a leak and with engine running, it is an audible sound, wouldn't call it a hiss, but a slight noise. Then you can shut the engine off, and the first press of the pedal, it seems to let all of the vacuum out, and it is done. I bet there is a small leak in the diaphragm, allowing vacuum into the atmospheric chamber Thanks for the advice.
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Laughing Gas
Joined: Aug 2012
Posts: 920
Likes: 20
From: Malheur Co. OR
I have lost the boosters in both my trucks now.
The '78 would be a hard pedal occasionally and feel like only the fronts were partially applied. I confirmed this by accident one day when I came to a stop on ice and the back tires were still spinning. When the leak suddenly became audible the truck also started running like crap.
The '79 suddenly started taking a lot of effort to press the pedal and will no longer idle. Since the heater controls are seized with us only reaching the 30s 3 times since the beginning of December and it is buried under two feet of snow, something that hasn't happened here since the mid 80s before I was born, I'm not worried about fixing it at the moment.
The '78 would be a hard pedal occasionally and feel like only the fronts were partially applied. I confirmed this by accident one day when I came to a stop on ice and the back tires were still spinning. When the leak suddenly became audible the truck also started running like crap.
The '79 suddenly started taking a lot of effort to press the pedal and will no longer idle. Since the heater controls are seized with us only reaching the 30s 3 times since the beginning of December and it is buried under two feet of snow, something that hasn't happened here since the mid 80s before I was born, I'm not worried about fixing it at the moment.
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Thread Starter
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Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 117
From: starship enterprise
Mine makes vacuum, then if you shut the engine off with pedal down, it just draws air in until it neutralizes, then you restart and vacuum comes back. I better order a new one.
Fleet Mechanic
Joined: Mar 2004
Posts: 1,398
Likes: 17
From: Hesperia, CA
Be very cautious about a leaking booster. About 20 years ago I had the Number 4 and number 7 intake valves warp on my 460. I took the heads to the machine shop to get the heads checked out and the old-timer at the machine shop told me to change the booster as it was leaking. He said the only cylinders that feed the vacuum to the booster are number 4 and 7 and if the booster is sucking air it pulls cold air into the two cylinders and can warp the intake valves. I went home and checked the intake manifold and he was right. I changed the booster and never warped another valve.
Junior User
Joined: Dec 2016
Posts: 96
Likes: 3
On my 79 when I push on the brakes the booster makes a noise which the easiest way to explain it is a farting noise. I have done the tests that I know of on it and it has passed all of the tests, pushing on the brakes then starting the engine and pedal goes down some etc. Could this booster still be defective causing my pedal to somewhat feel spongy?? I have seen elsewhere on the internet that a defective booster could make the pedal feel spongy but it did not come from a real reliable source. As much as I have bled the lines there is no way there is any air in the lines.