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Discussion Starter #1
I've been reading past posts on brake troubles all day, but I don't think I've found my answer.

When driving, the brake pedal will sink to the floor.
I just replaced the front rotors/pads which were in need, and bled the front brakes. Brakes still felt bad, so I bled the rears also.
No signs of brake fluid anywhere, I've followed the lines, and nothing coming from the drums. The fluid level does not drop.

When the engine is off, I have decent pressure in the pedal. When the engine is on, the pedal will sink to the floor. I cannot pump it up either.

My ideas were MC and soft lines (might use this as an excuse to upgrade the booster as well, but not sure if that is necessary)

any ideas?
 

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Bad bb/mc.
 

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I recently spent 3 days weekend chasing down the same exact thing.

Mine ended up being a bad MC. But it was still a beeyatch to bleed.

It can take a lot of fluid to bleed one of these. Just because you get clear fluid, doesn't mean you've worked out the air yet. Bleed in the correct order (RR,LR,RF,LF). Sometimes you can get a pocket in the proportioning valve that will only come out after pumping half a quart through the front system.
 

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there is another spot to bleed that ya'll forgot about. I forgot what it's called, but it's in the engine bay on the drivers side. You can se it easy while laying on the ground. I had to blead that out when my brake lines failed.
 

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there is another spot to bleed that ya'll forgot about. I forgot what it's called, but it's in the engine bay on the drivers side. You can se it easy while laying on the ground. I had to blead that out when my brake lines failed.
the proportioning valve? its attached to the frame on the Drivers side.. two lines run from the MC and go to the proportioning valve. from there the lines split for the two front, and single rear lines....
 

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One other spot that might need it if the vehicle is equipped is the rear ABS moducle that is inline with the rear brake line. Had the same issue and forgot to bleed it after doing all of the brakes....:doh0715:
 

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Discussion Starter #8
I was just thinking about that - the RABS issue. I never thought that I had RABS because I had never felt them engage. But I went under the truck yesterday, and sure enough, there is the valve/modulator/slave (whatever its called). I guess that could be the culprit too. My initial thought is to bypass it because I'm not really a fan of ABS anyways.
 

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Discussion Starter #9
So here is the update. I picked up a new MC, but I also grabbed a couple of plugs to test my MC first. I unhooked the brake lines and put in the plugs to test the MC. I pressed the brake pedal, and it was very firm. Then I started the truck for some vacuum help and it was still firm.
So I guess this means that the MC is okay - sound right?

So my next guess is the RABS unit. Or could the soft lines be the culprit? My rear soft line is awfully close to the exhaust piping.

Thanks for the help
 

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Driving Stuff Henry Built
-90 xlt, 351w, e4od, man 1356, 3.55, sag, warn hubs, 35s. -73, 400, np435, d20j twin, 35s
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Did you bleed the RABS unit like these guys said? It has a bleeder on it too (If the RABS isn't working because of another problem, the yellow abs light should light, & there are codes that can be pulled when it does).

How did you bleed the brakes? Here's a copy of the 2 man procedure if you're not familiar with it.

2 Man Brake Bleed
1-Usual safety stuff (Block the wheels, trans in correct gear, on safe axle stands if jacked up, etc.). -Broncos are high enough you should be able do do this without jacking it up.
2-Check the level of the brake fluid, add if needed. Recheck periodically & add as needed throughout the bleeding process. Do not allow it to run out. If it runs out, you must start over.
3-Have your helper pump the brake pedal a few times & then hold it down. Do not let the pedal up while the bleeder is open.
4-Open the bleeder valve, observe the fluid flowing from the bleeder. Fluid & air bubbles can spray, it's best to run a clear hose from the bleeder into a container. Some would say wear safety goggles. Keep your mouth closed :toothless.
5-Close the bleeder valve while the pedal is still depressed.
6-Have the helper pump the brake pedal, observe height of pedal.
7-Repeat steps 2-6 until no bubbles are seen in the fluid from the bleeder valve, then move to the next wheel, following this order on a 1990: RR, LR, RABS valve, RF, LF. Be sure that each bleeder is closed before moving on. If the brakes are otherwise in good condition, adjusted, & not leaking, the pedal should be firm when done.
8-Check the level of the brake fluid, add if needed. Recheck again soon to be sure fluid isn't leaking.
 

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Discussion Starter #11
So I bypassed RABS and decided to replace the soft hoses, which ultimately lead to replacing the entire front set of hard lines and one of the rear ones.

After all of that I bled the brakes exactly as El Kabong suggested above.

The pedal is hard (engine off), but if i continue to apply pressure to the pedal, it will eventually sink.

So ultimately, i think I have a bad Master Cylinder. I can't figure on how a bas caliper or rear wheel cylinder would cause this issue. Any thoughts?
 

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-90 xlt, 351w, e4od, man 1356, 3.55, sag, warn hubs, 35s. -73, 400, np435, d20j twin, 35s
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:stupid If the pedal creeps down without losing fluid, it's working past the master cylinder's own piston & the m/c needs to be replaced or rebuilt. A bad caliper or wheel cylinder causing the pedal to creep would show signs of leaking & the fluid level in the m/c would go down.
 

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Not to hijack but I am having some similar problems. I just did a sas and am working on getting the brakes working. I got rid of all the abs stuff and am running a center drop front and regular rear. No rabs valves or anything like that. I have bench bled the MC a ton and bled each wheel in order many times.
My problem is that when running the pedal goes all the way to the floor with no stopping, and if I keep pushing it will lock up the rear brakes. I can pump the brakes while rolling and it will brake fine, but after a few seconds it looses all pressure and the pedal goes to the floor again.
Any ideas of what it could be, I have no leaks and am running the F350 MC and booster.
 

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The rear brakes worked fine before and I didn't change anything with them. I've got the f350 wheel cylinders but they aren't leaking. What would cause me to have brakes after I've pumped the pedal, but the pressure goes away after a few seconds. I have taken the MC off numerous times and it is not leaking.
 

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The rear brakes worked fine before and I didn't change anything with them. I've got the f350 wheel cylinders but they aren't leaking. What would cause me to have brakes after I've pumped the pedal, but the pressure goes away after a few seconds. I have taken the MC off numerous times and it is not leaking.
I would look for a pressure leak at a higher point
 

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Driving Stuff Henry Built
-90 xlt, 351w, e4od, man 1356, 3.55, sag, warn hubs, 35s. -73, 400, np435, d20j twin, 35s
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I can pump the brakes while rolling and it will brake fine, but after a few seconds it looses all pressure and the pedal goes to the floor again.
Those symptoms (Loosing pressure, pedal dropping, & no fluid leaking) all point to a master cylinder leaking fluid past it's internal seals & back into the reservoir. You won't see any leaks or loose fluid because it returns to the reservoir.

When that type of leak 1st starts, it's usually more noticable with light pressure on the pedal, & then becomes worse, even with heavy pedal pressure. It's a wierd feeling, almost like it's working, then someone opens a valve somewhere, & the pedal goes away.

If it were leaking at any hoses, lines, calipers or wheel cylinders, it would leak to the exterior where you could find it. If it were air in the lines, the pedal would be spongy & would never feel good. So since it feels like a leak, but doesn't show a leak, look to the master cylinder because that's the only place where it can be loosing fluid back into itself.


I suppose another possibility is that something is hanging up in the rear brakes, then releasing as you feel the pressure loss. It seems unlikely, especially since your work did not involve the rears, but not impossible. It might be worth pulling the drums off & having a quick look. Check the backing plates for wear on the pads that the shoes slide against, & notice if the self adjuster cable is taut. If it's loose, it indicates that it's not adjusting.

And, as long as your checking stuff, make sure the bleeders are at the top, so the air that rises is bled off (People have gotten calipers swapped left for right & ended up with bleeders on the bottom). That doesn't sound like it fits your symptoms, but it's also worth a quick look.
 

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Well I replaced the MC but no change. I did use vice grips and close the center drop on the front, and with that done the rears work fine and the pedal feels fine. When I close off either one of the two front calipers it brakes fine.
Is it possible that the single center drop line can't push enough fluid to work both thunderbird calipers? I have bled the entire system many times and I just can't figure this out.
At least now I know the problem is in the front.
 

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What master cylinder are you using? Both the F350 rear cylinders and the T-bird calipers require more fluid to operate properly, a stock Bronco unit may not have enough fluid displacement.
 
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