History: Friend has 86 bronco, 4x4, inline 6. The truck has a vacuum issue where if pushes the brakes when its warm out it wants to bog and die. It also idles very high. We rebuilt and adjusted the carb with no luck. The vacuum lines are also in so so shape and several are blocked off we cant figure out where they go.

Issue:Whats going on is when he hits the brakes the drivers side rear tire (I assume drive wheel?) slows down but will not stop. We thought that the brakes needed to be adjusted but it only marginally helped-he can push the brake pedal with all his weight and it still moves and wont stop completely. The passenger tire stops no problem at all.

We were thinking aside from the engine issues that the rear wheel cylinders worn down and not pushing 100% anymore. Brakes feel fine-no air in the system from what we can tell either.

Ideas?

 

Aside from bleeding the entire brake system starting with the wheel furthest from the master cylinder, I would consider replacing the wheel cylinder at that wheel.

The engine bogging down when the brakes are applied should have nothing to do with vacuum. The supply of vacuum to the booster is constant. The high idle could be due to a leaking booster though. If the booster doesn't hold vacuum it is letting air into the engine from a location where it shouldn't. (Vacuum leak)

I don't have vacuum diagrams for that particular model/engine/year. You might want to see if you can track one of those down just to make sure your vacuum connections are complete and correct.

 

Yep.........Greystreak is right...............Sounds like a bad brake booster.......it's normaly the seals to the inside ......hear a sucking noise inside when you press the brake pedal ??

 

Thanks guys. I kept on telling him for about a year to swap the booster since its a rather easy job but hey sometimes people wont listen. Another friend had an idea that the booster was drawing too much vacuum from the engine cause of the leak causing it to idle weird and bog/die when the brakes were applied.

Were gonna replace both side wheel cylinders, if the booster doesnt cure it.

Thank you!

 

Wheel cylinders will not make it idle poorly when you hit the brakes !!......pull the vacuum hose off the booster and plug it off (the line not the booster)....hit the brakes.....bet the problem is gone.....

 

Funny, but we tried it and it made no change. But I keep telling him theres not much else that can cause issues with the bogging when you hit the brakes.

 

If you plugged the line from the engine to the booster.....it's not the booster.......don't know what you have other than a big electrical issue.......or an extremely weak alternator and/or battery that is pulling the voltage down below 12V's when the brake lights come on...highly unlikely !!!...........what happens when the headlights are turned on on ?? anything ?? is the brake pedal physically hitting any wiring harnesses under the dash when it is pushed ???

 

I dont think its electrical. There are some other issues too, see below.

I think is his vacuum setup is bad period. There are many plugged lines that go nowhere and broken lines. Also I told him the booster was shot and to try another but he refuses.

What also happens is when he hits the brakes driving normally the truck bogs and the idle goes crazy. While running no mater how much we adjust the carb if we put the idle to low it wont stay running either. Strangely the colder it is outside the less it bogs when you brake too, warm outside only way to come to a complete stop is to put it into neutral and feather the brake pedal.

So is it all perhaps related-im guessing so, im just confused to exactly whats really causing it.

 

Are we absolutely certain the problem is created when he applies the brake and not just when he releases the accelerator? Ford used a throttle kicker on a lot of older carbs to keep the butterflies from snapping shut when the pedal was released. Could he be dealing with something like that or was all that addressed when the carb was redone?

 

No Im pretty positive that was looked over when the carb was done.

Whenever he hits the brakes the truck bogs and then dies unless he shifts to neutral and feathers the brakes a bit. colder it is outside the less this happens, as the truck warms up and the weather itself gets warming he basically coasts till he has to apply the brakes fully.