1989 Bronco XLT Brake Issue

UhDabbler

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Hey… me again….

Few weeks ago I replaced my rear drums, shoes, and wheel cylinders. I bled the brakes but didn’t drive the truck because I was also doing transmission work. The brake pedal is firm while the truck is off. Now that the transmission work is done I started the truck up so I could finish filling my transmission fluid but when I put my foot on the brake the pedal went to the floor. After an hour of bleeding and 2qts of fluid I was still getting bubbles and the pedal isn’t firming up any. I did hear a distinct click from time to time from the rear brake lines while I was under there and my wife was pumping the brake pedal. Kind of at a loss, I don’t suspect the booster or MC and I don’t want to spend $200 on new parts to replace them and not solve the issue. I have a video of the bubbles that I can upload shortly.

Also the rear right gave me no bubbles, the rear left does. The image is the nasty fluid we flushed out.
 

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miesk5

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Yo UH,
  • Excessive Brake Pedal Travel,
  • & IF Brake Pedal Feels Spongy When Fully Applied
  • Rear brakes out of adjustment.
  • ADJUST rear brakes.
  • Front wheel bearing out of adjustment.
  • ADJUST front wheel bearings.
  • Brake master cylinder or power brake booster dash unit mounting loose.
  • TIGHTEN nuts and bolts to specification.
  • Front or rear disc brake caliper attachment loose.
  • REPLACE or TIGHTEN as required.
  • Worn or damaged self-adjusters.
  • REMOVE brake drum and CHECK rear brake shoe and lining for proper adjustment. REPLACE brake shoe adjusting lever.
  • Brake pedal or support improperly mounted.
  • REPAIR or REPLACE as necessary.
  • Worn or damaged brake linings.
  • REPLACE brake shoe and lining.
  • Brake drum out-of-round or front or rear disc brake rotors with excessive runout.
  • REFINISH or REPLACE if wear exceeds limits.
  • Worn or damaged RABS valve accumulator (F-Series only).
  • Brake tubing improperly positioned.
  • CHECK brake tubing for misposition near heat source. Fluid can boil and result in spongy pedal response.
  • Low brake fluid level.
  • FILL as required.
  • Clogged brake master cylinder filler cap hole.
  • REPAIR or REPLACE brake master cylinder filler cap.
  • Air in system.
  • BLEED system.
 

Tiha

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If you let the truck sit for a couple minutes and hit the pedal, does it go all the way to the floor, or go down slowly?

If you pump the pedal does it get firm? and Hold it? Or fall to the floor with your foot on the pedal?
 

UhDabbler

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Yo UH,
  • Excessive Brake Pedal Travel,
  • & IF Brake Pedal Feels Spongy When Fully Applied
  • Rear brakes out of adjustment.
  • ADJUST rear brakes.
  • Front wheel bearing out of adjustment.
  • ADJUST front wheel bearings.
  • Brake master cylinder or power brake booster dash unit mounting loose.
  • TIGHTEN nuts and bolts to specification.
  • Front or rear disc brake caliper attachment loose.
  • REPLACE or TIGHTEN as required.
  • Worn or damaged self-adjusters.
  • REMOVE brake drum and CHECK rear brake shoe and lining for proper adjustment. REPLACE brake shoe adjusting lever.
  • Brake pedal or support improperly mounted.
  • REPAIR or REPLACE as necessary.
  • Worn or damaged brake linings.
  • REPLACE brake shoe and lining.
  • Brake drum out-of-round or front or rear disc brake rotors with excessive runout.
  • REFINISH or REPLACE if wear exceeds limits.
  • Worn or damaged RABS valve accumulator (F-Series only).
  • Brake tubing improperly positioned.
  • CHECK brake tubing for misposition near heat source. Fluid can boil and result in spongy pedal response.
  • Low brake fluid level.
  • FILL as required.
  • Clogged brake master cylinder filler cap hole.
  • REPAIR or REPLACE brake master cylinder filler cap.
  • Air in system.
  • BLEED system.
This is incredible helpful!
 

UhDabbler

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If you let the truck sit for a couple minutes and hit the pedal, does it go all the way to the floor, or go down slowly?

If you pump the pedal does it get firm? and Hold it? Or fall to the floor with your foot on the pedal?
When the vehicle is off the break is extremely firm and wont push down at all without an amount of excessive force that makes me worried about damaging something. But When i put my foot on the break and start the truck the pedal just falls straight down, it will return to normal position once my foot is removed.
 

UhDabbler

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Here is the video of the bubbles. I did have the truck running as it appeared that the issue is exclusively when it is on.

xc_hide_links_from_guests_guests_error_hide_media
 

Tiha

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Those bubbles coiuld be coming around the threads of the bleeder wouldn't be too worried about that yet.

So the truck is running and the pedal goes to the floor.

If you pump the pedal does it get hard and hold pressure?
Or still go to the floor every time?
 

UhDabbler

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Those bubbles coiuld be coming around the threads of the bleeder wouldn't be too worried about that yet.

So the truck is running and the pedal goes to the floor.

If you pump the pedal does it get hard and hold pressure?
Or still go to the floor every time?
It’s still to the floor every time. I swapped my drums and shoes on the rear but I haven’t bled the R ABS valve yet because I figured I had avoided draining the MC too low.
 

Tiha

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If it is going to the floor every time and will not pump up I believe that is the master cylinder. Unless you have a leak.

The resistance you feel with the truck off is simply the booster without vacuum to assist.

Even a system full of air will eventually pump up to a firm pedal.
 

UhDabbler

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If it is going to the floor every time and will not pump up I believe that is the master cylinder. Unless you have a leak.

The resistance you feel with the truck off is simply the booster without vacuum to assist.

Even a system full of air will eventually pump up to a firm pedal.
I’m going to give it one more attempt at bleeding (going to vacuum bleed this time) with grease around the valves.

I’ve heard when bleeding the R ABS valve you have to remove the spring that’s in the unit, so you have any input in that.

If this bleed fails to remedy my issue I’ll be replacing my master cylinder and booster at one time.
 

Tiha

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I personally have never had to bleed the RABS valve. I have heard of others doing it, but in all my axle swap, disc brake swaps, wheel cylinder replacements. Never touched it. Never needed to.
 

UhDabbler

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I personally have never had to bleed the RABS valve. I have heard of others doing it, but in all my axle swap, disc brake swaps, wheel cylinder replacements. Never touched it. Never needed to.
Then I won’t worry about it. It’s a 32 year old truck that sat for the last 4 years so seals and gaskets and loads of other small things are bound to go out.

I really appreciate everyone’s constant advice and assistance. One day I’ll be able to share a positive post showing off all of the work and effort.
 

miesk5

96 Bronco 5.0
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Yo Uh,
Finally found this!
"The Fading Bronco Brake Pedal by Jeff Baskowitz
Vehicles: 1987 through 1998 Ford Bronco. All engines with Rear Anti-lock Braking Systems (RABS).
System: Brakes/ABS.
Symptom/Problem: Brake pedal fade, low pedal. The problem is an internal leak in RABS valve.
Background: A common complaint on Ford light duty trucks with the rear anti-lock brake system is that the brake pedal will fade while under steady pressure. In a conventional brake system this would point to an external fluid leak or an internal leak (bypass) in the master cylinder. Generally, one and usually two master cylinders have been replaced already, leaving one very frustrated technician. In this system there is one other possibility - the rear anti-lock brake valve. This valve is the electro-hydraulic proportioning valve located on the left-hand frame rail between the master cylinder and the rear brakes. This valve controls the brake fluid during an anti-lock stop by restricting or bleeding off rear wheel cylinder pressure.
To better understand what's happening, let's take a quick look at system operation. The RABS module continuously monitors rear wheel speed by means of a sensor installed in the rear differential. When the sensor signal to the module shows a rapid decrease in speed - indicating rear wheel lockup will occur - it energizes the isolation solenoid in the RABS valve. By doing this, the normally open valve is now closed, blocking off the line to prevent further brake fluid from entering the rear wheel cylinders. If there is still too great a decrease in rear wheel speed, the module will then energize the dump solenoid in the RABS valve. This solenoid is normally closed and, when energized, will open a port to the accumulator and bleed off some of the pressure to prevent lockup. This fluid is then stored until normal brake functions are resumed and the fluid is returned to the master cylinder.
If rear wheel speed is now OK, the module de-energizes these solenoids to resume normal braking. If speed decreases too rapidly again, it resumes anti-lock control. The module will alternate between the two solenoids and normal braking until there is no further chance of locking up or vehicle speed is now under 5 mph.
The focus is on this dump portion of the RABS valve. Occasionally the seat between the solenoid and the accumulator will leak, causing the pedal to fade during normal pedal applications. This sinking pedal has no other abnormal braking characteristics and is usually not noticed (depending on severity) unless it sits for an extended amount of time with constant pressure applied. It may be due to physical damage or a buildup of contaminants in the hydraulic system, but the results are the same.
Some customers report that the brakes were just fine until a hydraulic component was replaced or repaired and the brakes were bled. Then the pedal started fading. Quite possibly this is due to a new problem that is created by "moving" some of the contaminated fluid into the valve. To diagnose this system, like all brake systems, first verify that there are no external fluid leaks. If visual inspection is OK, disconnect the brake lines at the master cylinder and plug the outlets in the cylinder. When the master cylinder is plugged, if a fade is still experienced, the master is at fault.
If, however, there is no fade, determine if it is coming from the front or rear brakes by blocking one port at a time at the master cylinder. If there is no longer a fading pedal when blocking the port to the rear line, go one step further and build a short brake line to splice between the inlet and outlet lines at the RABS valve to temporarily bypass the valve.
After bleeding the rear brakes and verifying there is a solid pedal, it's a good bet that the problem is in the valve itself. The next step is to replace and bleed the valve (if equipped with a bleeder) and the rear wheel cylinders. Verify the repair and road test.
Jeff Boskowitz, IDENTIFIX Ford specialist, is certified Ford EEC, MACS, ASE master and L1. He has 26 years of experience."

And for future troubleshooting:
RABS Valve pic in 87-92 Bronco & 87-96 F Series
You must be registered for see images attach

by former member Steve83

Flashout Codes
Whenever the yellow REAR ABS light comes on during normal operation, a flashout code may be obtained to aid in problem diagnosis. If the vehicle is shut off before the code is read from a RABS-I module, the code will be lost. In some cases, the code may reappear when the vehicle is restarted. In other cases, the vehicle may have to be driven to reproduce the problem and, if the problem was associated with an intermittent condition, it may be difficult to reproduce. Therefore, whenever possible, it is recommended that the code be read before the vehicle is shut off.

WARNING: PLACE BLOCKS BEHIND THE REAR WHEELS AND IN FRONT OF THE FRONT WHEELS TO PREVENT THE VEHICLE FROM MOVING WHILE THE FLASHOUT CODE IS BEING TAKEN.

NOTE: If the red BRAKE light is also on, due to a grounding of the fluid level circuit (perhaps low brake fluid), no flashout code will be flashed and the REAR ABS light will remain on steadily.

NOTE: If there is more than one system fault only the first recognized flashout code may be obtained.

WARNING: PLACE BLOCKS BEHIND THE REAR WHEELS AND IN FRONT OF THE FRONT WHEELS TO PREVENT THE VEHICLE FROM MOVING WHILE THE FLASHOUT CODE IS BEING TAKEN.

To obtain the flashout code, locate the RABS diagnostic connector (with the Bk/Or wire), and attach a jumper wire to it. Momentarily ground it to the chassis. When the ground is made and then broken, the REAR ABS light should begin to flash.

NOTE: If the red BRAKE light was on (as noticed before the parking brake was applied), the problem may be with the low fluid level circuit and, in this case, no flashout code will be flashed and the light will remain on steadily.

The code consists of a number of short flashes and ends with a long flash. Count the short flashes and include the following long flash in the count to obtain the code number. For example, three short flashes followed by one long flash indicates Flashout Code Four. The code will continue to repeat itself until the key is turned off. It is recommended that the code be verified by reading it several times. In addition, the first code flashed may be too short because it may have been started in the middle. It should be ignored.

RABS Troubleshooting in 87-88 Broncos @ http://www.supermotors.net/registry/media/559308_1
Click NEXT to View more pages; miesk5 Note; for all Resistance tests, battery needs to be disconnected; remove Negative cable at battery
Source: by Jem270
RABS Self Test Diagnostic Connector Location pic in an 88;
You must be registered for see images attach

"...it is next to the brake pedal which this is a picture of and is C111; It has the one wire that is black/orange.."
Source: by Jem270


RABS Wiring Diagram in 87-89 Bronco & F series (Mitchell)
You must be registered for see images attach

Source: by equivalent (Beetlejuice) at SuperMotors.net

You must be registered for see images attach
 
Last edited:

UhDabbler

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Marysville WA
Yo Uh,
Finally found this!
"The Fading Bronco Brake Pedal by Jeff Baskowitz
Vehicles: 1987 through 1998 Ford Bronco. All engines with Rear Anti-lock Braking Systems (RABS).
System: Brakes/ABS.
Symptom/Problem: Brake pedal fade, low pedal. The problem is an internal leak in RABS valve.
Background: A common complaint on Ford light duty trucks with the rear anti-lock brake system is that the brake pedal will fade while under steady pressure. In a conventional brake system this would point to an external fluid leak or an internal leak (bypass) in the master cylinder. Generally, one and usually two master cylinders have been replaced already, leaving one very frustrated technician. In this system there is one other possibility - the rear anti-lock brake valve. This valve is the electro-hydraulic proportioning valve located on the left-hand frame rail between the master cylinder and the rear brakes. This valve controls the brake fluid during an anti-lock stop by restricting or bleeding off rear wheel cylinder pressure.
To better understand what's happening, let's take a quick look at system operation. The RABS module continuously monitors rear wheel speed by means of a sensor installed in the rear differential. When the sensor signal to the module shows a rapid decrease in speed - indicating rear wheel lockup will occur - it energizes the isolation solenoid in the RABS valve. By doing this, the normally open valve is now closed, blocking off the line to prevent further brake fluid from entering the rear wheel cylinders. If there is still too great a decrease in rear wheel speed, the module will then energize the dump solenoid in the RABS valve. This solenoid is normally closed and, when energized, will open a port to the accumulator and bleed off some of the pressure to prevent lockup. This fluid is then stored until normal brake functions are resumed and the fluid is returned to the master cylinder.
If rear wheel speed is now OK, the module de-energizes these solenoids to resume normal braking. If speed decreases too rapidly again, it resumes anti-lock control. The module will alternate between the two solenoids and normal braking until there is no further chance of locking up or vehicle speed is now under 5 mph.
The focus is on this dump portion of the RABS valve. Occasionally the seat between the solenoid and the accumulator will leak, causing the pedal to fade during normal pedal applications. This sinking pedal has no other abnormal braking characteristics and is usually not noticed (depending on severity) unless it sits for an extended amount of time with constant pressure applied. It may be due to physical damage or a buildup of contaminants in the hydraulic system, but the results are the same.
Some customers report that the brakes were just fine until a hydraulic component was replaced or repaired and the brakes were bled. Then the pedal started fading. Quite possibly this is due to a new problem that is created by "moving" some of the contaminated fluid into the valve. To diagnose this system, like all brake systems, first verify that there are no external fluid leaks. If visual inspection is OK, disconnect the brake lines at the master cylinder and plug the outlets in the cylinder. When the master cylinder is plugged, if a fade is still experienced, the master is at fault.
If, however, there is no fade, determine if it is coming from the front or rear brakes by blocking one port at a time at the master cylinder. If there is no longer a fading pedal when blocking the port to the rear line, go one step further and build a short brake line to splice between the inlet and outlet lines at the RABS valve to temporarily bypass the valve.
After bleeding the rear brakes and verifying there is a solid pedal, it's a good bet that the problem is in the valve itself. The next step is to replace and bleed the valve (if equipped with a bleeder) and the rear wheel cylinders. Verify the repair and road test.
Jeff Boskowitz, IDENTIFIX Ford specialist, is certified Ford EEC, MACS, ASE master and L1. He has 26 years of experience."

And for future troubleshooting:
RABS Valve pic in 87-92 Bronco & 87-96 F Series
You must be registered for see images attach

by former member Steve83

Flashout Codes
Whenever the yellow REAR ABS light comes on during normal operation, a flashout code may be obtained to aid in problem diagnosis. If the vehicle is shut off before the code is read from a RABS-I module, the code will be lost. In some cases, the code may reappear when the vehicle is restarted. In other cases, the vehicle may have to be driven to reproduce the problem and, if the problem was associated with an intermittent condition, it may be difficult to reproduce. Therefore, whenever possible, it is recommended that the code be read before the vehicle is shut off.

WARNING: PLACE BLOCKS BEHIND THE REAR WHEELS AND IN FRONT OF THE FRONT WHEELS TO PREVENT THE VEHICLE FROM MOVING WHILE THE FLASHOUT CODE IS BEING TAKEN.

NOTE: If the red BRAKE light is also on, due to a grounding of the fluid level circuit (perhaps low brake fluid), no flashout code will be flashed and the REAR ABS light will remain on steadily.

NOTE: If there is more than one system fault only the first recognized flashout code may be obtained.

WARNING: PLACE BLOCKS BEHIND THE REAR WHEELS AND IN FRONT OF THE FRONT WHEELS TO PREVENT THE VEHICLE FROM MOVING WHILE THE FLASHOUT CODE IS BEING TAKEN.

To obtain the flashout code, locate the RABS diagnostic connector (with the Bk/Or wire), and attach a jumper wire to it. Momentarily ground it to the chassis. When the ground is made and then broken, the REAR ABS light should begin to flash.

NOTE: If the red BRAKE light was on (as noticed before the parking brake was applied), the problem may be with the low fluid level circuit and, in this case, no flashout code will be flashed and the light will remain on steadily.

The code consists of a number of short flashes and ends with a long flash. Count the short flashes and include the following long flash in the count to obtain the code number. For example, three short flashes followed by one long flash indicates Flashout Code Four. The code will continue to repeat itself until the key is turned off. It is recommended that the code be verified by reading it several times. In addition, the first code flashed may be too short because it may have been started in the middle. It should be ignored.

RABS Troubleshooting in 87-88 Broncos @ http://www.supermotors.net/registry/media/559308_1
Click NEXT to View more pages; miesk5 Note; for all Resistance tests, battery needs to be disconnected; remove Negative cable at battery
Source: by Jem270
RABS Self Test Diagnostic Connector Location pic in an 88;
You must be registered for see images attach

"...it is next to the brake pedal which this is a picture of and is C111; It has the one wire that is black/orange.."
Source: by Jem270


RABS Wiring Diagram in 87-89 Bronco & F series (Mitchell)
You must be registered for see images attach

Source: by equivalent (Beetlejuice) at SuperMotors.net

You must be registered for see images attach
Thank you so much for this! I haven’t had time to dive back into my brakes yet. I have a checklist to go through to troubleshoot this and I’ll ad the RABS procedure as you’ve listed above.
 

GunnySutton

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Hey… me again….

Few weeks ago I replaced my rear drums, shoes, and wheel cylinders. I bled the brakes but didn’t drive the truck because I was also doing transmission work. The brake pedal is firm while the truck is off. Now that the transmission work is done I started the truck up so I could finish filling my transmission fluid but when I put my foot on the brake the pedal went to the floor. After an hour of bleeding and 2qts of fluid I was still getting bubbles and the pedal isn’t firming up any. I did hear a distinct click from time to time from the rear brake lines while I was under there and my wife was pumping the brake pedal. Kind of at a loss, I don’t suspect the booster or MC and I don’t want to spend $200 on new parts to replace them and not solve the issue. I have a video of the bubbles that I can upload shortly.

Also the rear right gave me no bubbles, the rear left does. The image is the nasty fluid we flushed out.
I would replace the Master cylinder. Sounds like one of the seals might have torn. If the power booster was bad you would hear it by the pedal. Don't forget to bleed from back to front.
 

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