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Brake pedal goes to floor with engine running

85b2denver

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First off I have a 1985 Bronco II 2.8l 5 speed with a 4 inch sus lift and 31 inch tires.
So I have been trying to diagnose this problem since i got a new mc/power booster combo wondering if the ones I got are faulty or not. Now, I have bled the system multiple times and im pretty sure there are no leaks, I have also replaced the rear wheel cylinders.
Now if I get in the car with it off, I can pump the brakes and they'll hold pressure. But as soon as I turn on the car, and it's not moving, the pedal drops to the floor immediately with no pressure. Now if i pump the brakes ridiculously fast, I can feel a tiny, but barely, amount of low end pressure build up that disappears as soon as I stop pumping, once again while the car is running but standing still. Me and my dad dont see any leaks, the brake fluid level hasn't dropped at all, and the check valve is working.
Another thing I notice in traffic, is if i have the pedal to the floor and I let up a little bit, the engine revs increase a very small amount. Sign of a bad diaphragm? Or is it normal due to less resistance to the vacuum pressure?
And one last bit of info, when the engine was running, we disconnected the line running to the power booster, the brake pedal was very firm, hopefully showing that the lines dont have air in them.

Edit: I forgot to mention that before I replaced the booster/mc combo due to hissing noises, the brakes felt to be adjusted and grabbing fine, so I don't think that should be an issue

Any help is greatly appreciated!!!!!
 
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Ewgoetz

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Air still in the hydraulic brake system.
Did you bench bleed the mc before you installed it?
 

85b2denver

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I have bench bled it, i changed out the old rear brake lines then i test drove it, the bottom end has improved, i did at least ten pumps per wheel cylinder all clear, no bubbles. Another thing i noticed is when i slam on the brake pedal fast, i feel resistance, when i slowly hit it, theres no resistance.
 

Ewgoetz

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I still think there is air in the system, if there are no leaks. Than it’s possible you may have gotten a defective mc.
Where did you get the mc from? Hope not autozone.
 
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Sounds like the same problems i had with my 87 BII . It turned out to be the NEW brake booster.
 
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