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Brakes - car OFF brake pedal firm, car ON brake pedal sooooft.. why???

matus2

matus2

NAXJA Forum User
Location
Phoenix, AZ
Ok so here is the entire story. Recently I purchased a used d35 with yukon shafts and a lockright locker. well, we swapped it for my 8.25 as a temporary while I rebuild it and some interesting problems appeared.

First the d35 is from a jeep other than an xj for the parking brake lines are not the length they should be. instead of both being the same length one is shorter than the other... Wondering if I can take out the parking brake lines from the 8.25 and use them for the time being.. worst comes to worst I can live w/o a parking brake for a while...

Second and this is the major problem, I have bled the brakes both front and rear to the point where there is only new fluid in the system and definately no bubbles... When I am in the car and engine is not running the pedal is firm. however when I turn the engine on the pedal goes soft, all the way to the floor... the rear does not seem to brake. also the axle came with abs which I first just ziptied out of the way and later played around with it hoping that it was the culprit but it wasnt...

So to recap, engine off pedal firm, engine on pedal soft. why?
 
JJacobs said:
Bad vacuum booster.
Click to expand...

No, good vacuum booster, bad air in the lines, calipers, wheel cylinders somewhere.

Make sure that all your bleed screws are on top and you have no leaks.

You have air somewhere.

Also, make sure your rear shoes are adjusted out to the drums. If they aren't, the pedal will use up all of it's travel getting them in contact.

Check bleed screws, leaks, adjust your shoes.
 
I'd adjust the shoes first. If that's not it, it probably has bad wheel cylinders. Mine went bad and caused the same exact symptoms. Same thing just happened to a friends ZJ too (old ZJ with drums).

Yes, you can use the 8.25 e-brake cables.
 
mrblaine said:
Also, make sure your rear shoes are adjusted out to the drums. If they aren't, the pedal will use up all of it's travel getting them in contact.
Click to expand...

Bingo. He tried to rig something temporarily to adjust them but it didn't work. I guess he gets to mess with the parking brake lines sooner than he thought

The booster should be fine, unless we managed to break it working on the rear axle :D
 
JJacobs said:
Bad vacuum booster.
Click to expand...
Read the problem. It is a bad vacuum booster as stated. Your booster should hold a vacuum for a couple of pumps after the engine is shut off. That's one way to find a bad brake booster.

He stated that his brakes are hard to push when the XJ is off, and easy to push when it was running.
 
Last edited:
The other problem he has could be the other things suggested, and possibly the master cylinder. I'd guess there to still be air somewhere. Did the master cylinder get drained and rebled? If so, you'd be good to bleed the mc first.
 
Every time I've had a brake booster go bad, you can hear a vacuum leak in it.

I seriously doubt this is the problem, and the booster simply assists in braking anyway, It wouldn't cause the pedal to hit the floor even if bad.

The master cylinder will do that, but the likely culprit is the wheel cylinders because by swapping the axle, you have done nothing to change anything else in the system besides rear brakes. It's gonna be in there somewhere.
 
Thank you guys for all the replies. I'll start by taking the parking brake cables from the 8.25 and fitting them to the d35 so I can adjust the shoes. Mine is a 91 and it could definately use a new brake booster anyways.. if all that doesnt work I'll move on to the wheel cylinders. Thank you all for helping out.
 
I wouldn't worry about your booster just yet. You should make sure that when it's running that your pedal doesn't go all the way to the floor. That issue needs to be addressed first. The vacuum booster failing will not make your pedal go to the floor. You have two seperate issues here.

Now, a bad Master Cylinder can do that, but if it worked fine before you swapped, it should still work unless you let the resivoir go empty when you changed axles. If so, you might need to bleed your MC.

If your bleeding your brakes the old two person manual way, you might want to get a bleeder kit that runs a tube from the bleeder screw into a storage bottle. That way seems to get a bit more air out, and you'll be able to use the fluid you are pushing out of it again.
 
matus2 said:
Thank you guys for all the replies. I'll start by taking the parking brake cables from the 8.25 and fitting them to the d35 so I can adjust the shoes. Mine is a 91 and it could definately use a new brake booster anyways.. if all that doesnt work I'll move on to the wheel cylinders. Thank you all for helping out.
Click to expand...
you shouldn't NEED the parking brake cables to adjust the shoes, it's just that the automatic adjuster won't adjust automatically without the parking brake.
frankly they work like poo anyway.

Pull the rubber plug out of the back of the brake backing plate, right through that hole is the brake adjsut star wheel.

Turn until you feel a slight drag on the brake drum when you spin it.

have beer
 
87manche said:
you shouldn't NEED the parking brake cables to adjust the shoes, it's just that the automatic adjuster won't adjust automatically without the parking brake.
frankly they work like poo anyway.

Pull the rubber plug out of the back of the brake backing plate, right through that hole is the brake adjsut star wheel.

Turn until you feel a slight drag on the brake drum when you spin it.

have beer
Click to expand...

It sounds like he has the parking brake cables BS'd in there and they are holding a bit of tension on the spreader bar.
 
The symptons were the same before and after he attempted to put tension on the unconnected parking brake cables. They aren't dragging.

I'd say the drums need adjustment and if that doesn't resolve the issue it will likely be the wheel cylinder(s).

Thanks for the tip on adjusting the drums without the parking brakes. It's been so long since i've looked at drums I had forgotten about that completely.

Thanks for the help
 
I was just going off the stated concern. Things can be a little hard to diag over the internet.

Here's a good park brake adjustment check- with the proper light drag dialed into the rear brakes, remove the drums. The park brake strut that goes just over the axle shaft should have a little front to back free play, kept taut by the spring on the end of the strut. If it's bottomed into the shoes front and rear then the cables are too tight.
 
JJacobs said:
I was just going off the stated concern. Things can be a little hard to diag over the internet.

Here's a good park brake adjustment check- with the proper light drag dialed into the rear brakes, remove the drums. The park brake strut that goes just over the axle shaft should have a little front to back free play, kept taut by the spring on the end of the strut. If it's bottomed into the shoes front and rear then the cables are too tight.
Click to expand...

That's not a bad way to diagnose things, but on higher mileage rigs, a groove gets worn by the friction material and it leaves a ridge that prevents easy removal of the drum without backing off the shoes.
 
As stated by mrblaine. Make sure the bleed screws are on the top. They will not bleed all the air out if they are not. "I KNOW".
 
If the new rear axle has different brakes, the master cylinder may not have the capacity to supply them. If the wheel cylinders on the new axle are bigger than the old ones they require more fluid volume to work. You may need a bigger bore master cylinder. The brakes may be hard with engine off due to no vaccum at the booster. The pedal should not go to the floor.
 
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