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Discussion Starter #1
First off I would like to say "thank you!" to everyone posting here. The information on this site has helped me though my 4BT swap journey.

(1994 4X4 Toyota standard cab swap)

I am in the final stages of my 4BT swap and ran into an issue with the brakes. I am running the 6BT powersteering/vacuum pump with the factory Toyota brake booster. After testing all the components from the confined space of the backyard I felt it was safe enough to take onto the street for a short drive. All seemed well until I had to brake! The pedal was extremely hard to press making for a difficult stop. During the, almost two hour, test in my backyard everything seemed fine. The only variable I can think of is the amount of speed, both engine rpm and linear motion. After making back home I checked to see if the pump or booster failed. The pump seems to be working just fine (without have a gauge to read exact measurements). This leads me to think that the diaphragm may have been damaged by excessive vacuum.

Does anyone know if this pump may be applying too much vacuum for this booster? I don’t want to replace the booster just to have a new one fail. I have searched all over the web for more information regarding acceptable vacuum amounts but came up with nothing.

Kind regards,
Ryan Lanier
 

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Doubt the pump could draw enough to damage the booster.
Did you check for vacuum at the booster check valve?
If you pump the brakes several times with the engine off then holding the pedal down start the engine does the pedal go down a little as the vacuum comes on?
Have you had the booster/master cylinder apart in the conversion process? Maybe incorrect spacing on the booster to cylinder push rod or pedal to booster rod.
Cheers Steve
 

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Discussion Starter #3
Hi Steve -
This is great news. I was really worried that the pump was supplying too much vacuum.
I have not checked for vacuum at the booster check valve. I think the one that was on the truck originally was "inline" between the gas engine intake plenum and booster. That I am aware of I do not even have one, unless its internally located in the booster itself. I have the vacuum line coming right from the pump to the booster.
I will have to run that test once I have the radiator back in (I am redesigning the core support to allow for a larger fan on both the condenser and radiator). I am not sure of running it to long without a cooling system.
No, the booster and master have been together the entire time during the conversion process. However, I will make sure this spacing has not gone out of adjustment.
Thanks again!
 

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The check valve is usually the plastic elbow that plugs into the booster on the end of the vacuum hose.
There should be another one in the fitting going into the pump.
Was not worried about the check valve operation as much as that there is actually vacuum at the booster.
Cheers Steve
 

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You could check your vacuum booster by running a hose from a different car to your booster and push your Toyota brake pedal and see if it is easy to push. Your symptoms sound like you have little or no vacuum from your pump. I believe that check valve just keeps the vacuum in the booster.
 

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Discussion Starter #6 (Edited)
So its affirmative, my booster has gone bad. Using a Mityvac I was able to determine the booster will not hold vacuum. I am now leaning towards my pump as the root cause. The Mityvac shows a vacuum reading of 30 in at IDLE. This is extremely high, at roughly quarter throttle it buries the gauge (maximum gauge level) with a reading of 40 in. Does anyone know how to control this vacuum level? Most automotive applications that I have ever dealt with only require around ~15-20 in.

Just to sum it up again, this is a new 4/6BT style powersteering/vacuum pump.
 

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Think your gauge maybe off a bit, about 30" of Mercury is the maximum vacuum achievable.
I am certain that a booster in good condition would survive that, they would have to be designed to allow for maximum vacuum.
A good brake shop should be able to confirm what they can handle.
Cheers Steve.
 

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Yes, that vacuum pump is only capable of so much. Never known one to have too much vacuum. Which style pump does your setup have. There was an early one with 2 pumps and 2 different later styles. It can be a good idea to rebuild those later styles and kits are not that expensive. Here's a great video prepared by the company here in Murphy NC that is the king of those pumps. Well worth the hour it takes to watch it. https://www.youtube.com/watch?v=vJhR48a-9R8
 

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Discussion Starter #9
Thank you everyone who has replied. Since my original post (08-14-2017) the issue has been resolved. I installed a new booster (used) and a vacuum regulator set at 15", so far so good.
 
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