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Discussion Starter · #1 ·
Ok I didn't want to post this here as it should be a very simple problem and if it needs to be moved to newbie go ahead; I just didn't want a bunch of idiots asking if I bled the brakes but it is starting to piss me off and I need my DD back so here it goes: 93 YJ, stock axles for now, 33s, new rotors, pads, drums, good shoes, new MC, all new fluid (flushed out the old stuff), everything has been bled thoroughly and is adjusted up right. When the Jeep is not running I have a great pedal and it holds pressure fine, no sponginess either. As soon as I start the Jeep the pedal drops ALOT, not the inch or so it should, the brakes still work but the pedal has way too much travel before you can feel it working. I pulled the cap off the MC and pushed down the pedal to confirm that it is moving the MC piston by the bubbles and it does move the piston both with the engine running and not running. I know everything is fine mechanically with the brakes, my concern is why the pedal drops so much when the engine is on and why there is so much travel before the brakes have any feel? Is it possible that the booster could have something wrong internally which would cause this? I have only had a bad booster make the pedal too hard and not give any boost, never give too much??? I'm about ready to yank the whole damn system out and swap over to some real brakes on this thing.
 

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i would think the lines might be expanding when u push it hard, wouldnt be enough power when engine is off to do it so it should feel nice and solid, but when you hammer it with the engine running it goes squishy... makes me think the lines are getting old or have been stretched out repeatedly during flex and therefore expand easily now
 

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If you search some you may find a few good tech articles on adjusting the push rod length. Your new MC may not be exactly the same distance from the one you junked. It's really simple to lengthen the rod (it has an adjuster nut and screws out. you have to pull the MC back off the firewall to get at it.). Try making it a little longer. If that doesn't help you may have a bad new MC. It's happened before.
 

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Discussion Starter · #4 ·
Yeah I have adjusted the pushrod between the booster and MC till it makes the front brakes drag and it while the pedal is great when the Jeep isn't running it sucks as soon as I start it. I have been looking at exploded views of boosters and how they work and am wondering if maybe the internal spring is no good or the diaphragm has deteriorated causing this, it is probably the original with over 200k miles on it so...
 

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I would not rule out the rest of the system before that. My pedal is nice and stiff until I start the motor then the pedal goes to mush. If I plug the rear brake lines my pedal becomes stiff and I can lock up the front brakes witht he motor on. Something is going on with my rear lines and/or calipers. I given up on air in my lines due to bleeding 6 large bottle of brake fluild through the system by gravity, pump, and pressure bleeding them.
 

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ftrain said:
I would not rule out the rest of the system before that. My pedal is nice and stiff until I start the motor then the pedal goes to mush. If I plug the rear brake lines my pedal becomes stiff and I can lock up the front brakes witht he motor on. Something is going on with my rear lines and/or calipers. I given up on air in my lines due to bleeding 6 large bottle of brake fluild through the system by gravity, pump, and pressure bleeding them.
...or the master cylinder. ;)
 

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Why would you think the master if I am plugging the lines after it. First I plugged the master, then I plugged after the partion valve, and then at the "T" in the rear axle. All have provided me with a firm pedal. I was ruling everything out before the plug since the pedal was firm. I keep thinking there is air in my cadi calipers and keep removing them from the bracket to bleed them some more. It kills me because I read all over the place people have been able to get the saem exact brake setup as mine to work. 68 Corvette MC, YJ booster, GM brakes up front, caddi calipers in the rear.
 

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When you have a pedal with the engine off and not when the vehicle is running it's one of two problems.

1. Air
2. Master cylinder is too small.

Seen this happen 1000s of times.
 
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