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brake booster issue


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#1 herculesinwyoming

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Posted 20 July 2010 - 05:56 PM

i have a 76 datsun 620 that had sat for about 20 years, when off i get a decent brake pedal, but if the truck is running the brake will go strait too the floor, i am thinking the check valve for the booster is bad because it vents through the vacum line when it is off and i push the pedal,
but how do i test the booster too? any ideas or sugestions, i am just trying to avoid buying parts i dont need

#2 datzenmike

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Posted 20 July 2010 - 06:23 PM

Sounds fine to me. The booster simply takes your foot pressure and magnifies it to increase the brake pressure. It uses engine vacuum to do this.

With engine off the peddle feels stiff or firm because you have no help from the booster. With engine on, the booster will increase your input several times over and of course the peddle will drop down some. If you system is working properly, pump the peddle several times with engine off. Push down on peddle firmly and turn engine on.... peddle should drop slightly. Everything is fine. Peddle dropping to the floor is not.

The one way valve allows the engine to draw air from the booster into the intake. The booster stores this vacuum and when the motor is shut off, the one way valve prevents air from getting back in. If your engine is off and you pump the brakes you will hear the vacuum being used to assist the brakes. 2-3 pumps and it is all used up.


You have 4 wheel drum brakes so you should adjust the shoes out closer to the drums. Right now most of the peddle travel is wasted moving all the shoes out toward the drums.
Bleed the brakes. There may be air trapped in the lines and 20 year old brake fluid is a no no.

#3 thisismatt

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Posted 20 July 2010 - 06:24 PM

Even with a booster you're still physically connected through to the MC when you push on the pedal, so unless something is broken inside the booster (not likely) then I'd still lean towards your having a MC, air in the lines, or other brake system problem.

I have not yet begun to defile myself.


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#4 Pumpkn210

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Posted 20 July 2010 - 07:20 PM

but how do i test the booster too? any ideas or sugestions, i am just trying to avoid buying parts i dont need



You have 4 wheel drum brakes so you should adjust the shoes out closer to the drums. Right now most of the peddle travel is wasted moving all the shoes out toward the drums.
Bleed the brakes. There may be air trapped in the lines and 20 year old brake fluid is a no no.

Bleed the brakes and flush out all the bad fluid and air. After you have done that, you can see how it acts. My guess is that it has bad fluid/air in the lines. And after sitting for that long it should be replaced.
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#5 herculesinwyoming

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Posted 20 July 2010 - 07:27 PM

i have brand new master, all new wheel cylenders, brakes work when the truck hasnt been running, but when i start it the brake pedal goes to the floor with zero pedal feel, and no brakes.
i have bled the brakes multiple times, and with the engine running the brakes wont work

#6 Pumpkn210

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Posted 20 July 2010 - 07:32 PM

sounds like the booster is the only thing you havent changed. So... what are you waiting for ;)
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#7 thisismatt

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Posted 20 July 2010 - 07:42 PM

They work, or they feel like they're working?

I have not yet begun to defile myself.


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#8 herculesinwyoming

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Posted 20 July 2010 - 08:48 PM

they stop the truck, any way i will get after the issue in the morning, i have to be missing a small issue, maybe i just have a bad dry rotted vacum line or such

#9 thisismatt

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Posted 20 July 2010 - 09:07 PM

Your pedal should never go to the floor is the bottom line. Air in the lines, brakes out of adjustment, leaks, bad MC seals, failing-but-not-leaking soft lines (ballooning)...any of these could allow the pedal to go to the floor. With the engine off, how much pressure are you applying to see if the pedal drops excessively far? What if you really stand on it?

I have not yet begun to defile myself.


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#10 I'm BLUE

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Posted 20 July 2010 - 10:42 PM

Does the Brake Booster have adjustable rod ? If so did you take attention to not position or adjustment when installing Brake Booster by chance ( hard for me to dial in some times on other model cars :huh: )

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#11 I'm BLUE

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Posted 20 July 2010 - 10:49 PM

hmmmmmmmm All of the Above Advice ^^^ Seems to be Solid.

If you really feel that the check valve is bad ( it has turned 2way instead of 1way ) , my suggestion would be to simply straight line it to the intake , and see if there is a difference between ----> having your current check valve and a straight vacuum hose. ---> If you don't see a difference you will have a problem with either the check valve or booster <--- ( At that case I would try or buy another check valve , have one from a spare truck or friend to borrow )

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#12 herculesinwyoming

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Posted 21 July 2010 - 06:33 AM

thanks for the tips, i will check it out this morning a bit closer, like i said nice firm pedal untill i start the truck, which just is a problem i have never seen before, i will figure it out today!

#13 DISLEXICDIME

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Posted 21 July 2010 - 06:56 AM

Un plug the booster cap of the vacummm line and see if it still does it.
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#14 herculesinwyoming

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Posted 21 July 2010 - 08:13 AM

after doing some testing on it today, all your ideas are good.

i am thinking the old flexable brake lines need replaced, and/or maybe it got some air in the system again, but i am leaning towards the flexable lines, and they probably should be changed either way, so i will get them and bleed it all again, i bet that fixes it.

the check valve works and the booster is holding a vacum, unless theres another part in the booster i dont understand.

i will have to order the brake hoses so it may be a while before i have an update.

Thanks for all the input

#15 thisismatt

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Posted 21 July 2010 - 03:56 PM

The booster is pretty simple, and I don't think there's anything that could fail that would make it behave differently with or without vacuum, other than the normal change in difficulty when pushing the pedal.

I have not yet begun to defile myself.


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#16 herculesinwyoming

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Posted 21 July 2010 - 04:03 PM

even way out here in wyoming i was able to order new flex lines that will be here tomorrow, after the job i will bleed them just like the book says to. i think im on the right track

#17 datzenmike

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Posted 22 July 2010 - 07:39 PM

If the one way valve were reversed no vacuum can get to the booster and there would be no power assist. Think about it.

The booster either works or it does not. Without power assist the brakes will work but the effort is high. With the booster working the brakes will work and are much easier to apply. You say that the brakes work with motor off and don't work when it is on.... this is impossible.

How can you say the brakes don't work with the engine running? Did you drive it? Or are you assuming because the pedal drops it isn't working? I already said the pedal would be stiff without booster assist and the pedal softer with it.

The biggest cause of excess pedal travel is trapped air OR the drums not adjusted.

You have 4 wheel drum brakes so you should adjust the shoes out closer to the drums. Right now most of the peddle travel is wasted moving all the shoes out toward the drums.
Bleed the brakes. There may be air trapped in the lines and 20 year old brake fluid is a no no.


Did you adjust the drums yet or are you assuming they are ok? If bleeding, don't forget the NLSV (load sensing valve) under the pass seat.

#18 herculesinwyoming

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Posted 23 July 2010 - 07:30 AM

yep i know i need to bleed the load sensing valve, everything checks out good on the booster, so i moved on to checking over the adjustment of drums and i am replaceing the flex lines and then bleeding the whole system just like the haynes manual says, but sadly i am working next few days so it will be a bit before i get it finished

#19 herculesinwyoming

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Posted 31 July 2010 - 09:18 AM

i have it fixed now, rear drums needed adjusted, and the flex lines were bad, and after another bleeding of the brakes i have a good firm pedal

#20 thisismatt

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Posted 31 July 2010 - 09:45 AM

good to hear

I have not yet begun to defile myself.


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