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1992 F-250 weak rear brakes
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franknbeans
New User
Apr 13, 2010, 7:04 PM
Post #1 of 6
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1992 F-250 weak rear brakes
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I have a 1992 F-250 4x4 with rear wheel ABS that I am stumped with. About a week ago my rear brakes seemed to disappear. The brake pedal got spongy and seems to only have stopping power up front. (on gravel front tires slide on hard application)
Checked underneath and found a leaking brake line behind the cab in the frame rail, so I patched in a new line and bled the entire system. No change. I adjusted the rear brakes and bled system again starting RR, LR. ABS valve, RF, and LF.--No change. During bleeding with a buddy in the cab he noted that pedal only goes half way down when I tell him that the fluid has stopped coming out of the bleeder. Not sure if this is normal??? but fluid comes out clear (no bubbles). With no luck I thought I would try a new master cyl. Bench bled unit, hooked it up and rebled entire system. NO CHANGE! Pedal seems firm with engine off and only 1/4 pedal when running--a little less than I expected. I do realize that the vacuum booster will drop it down, but I didn't think quite that far? No ABS light or brake light illuminated. I am stumped!
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Apr 14, 2010, 2:03 AM
Post #2 of 6
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Re: 1992 F-250 weak rear brakes
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Just a couple thoughts:
Rear line rusted out as they do too often. That alone should have quickly given you the RED brake warning light of an imbalance noted at the proportioning valve.
I think same bulb is used for parking brake. Rare to me but a proportioning valve can stick to one side locking out either front or back and illuminate the light. Light may not work and it's not consistent with you ability to bleed rear brakes as if it stayed stuck they shouldn't be bleeding out but there may be a strange fault in that somehow,
T
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franknbeans
New User
Apr 14, 2010, 5:22 AM
Post #3 of 6
(6174 views)
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Re: 1992 F-250 weak rear brakes
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You talk about a proportioning valve. I am familiar with those on some older vehicles that I have worked on, but have not found such a critter on this truck. There are two brake lines coming out of the master cyl. The front line drops down and goes directly to the front calipers, and line back line drops down and goes to the ABS valve and then feeds the rear wheels. No sign of a proportioning or centering valve in the system that I have found. I thought it might be incorporated into the master cyl. which led me to replace that with no success. I haven't seen a brake light on, but I will check it today by trying the e-brake. Thanks for the reply.
Frank
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Apr 14, 2010, 5:46 AM
Post #4 of 6
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Re: 1992 F-250 weak rear brakes
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Frank,
I don't know the trucks as well as cars as they rarely fit well in my shops over the years so I tried to avoid too much on them.
No matter - there has to be some system for "proportioning" forces which may be hidden inside the ABS monster - dunno. Generally many were "combination" valves to both proportion and to set of light if imbalanced enough. To center them when they used to stick (long retired now) you would bleed out just a bit from the end that worked right at the master cylinder.
Some Fords you couldn't bleed or recenter till you pushed a red button on the things which are spring loaded to self center but don't always.
Things that should/could trigger RED brake warning light: Low brake fluid in some, most now, unequal pressures noted, AND that parking brake - use same bulb in many vehicles despite manufacturer.
Trucks have a unique issue cars don't always address. You (the manufacturer) doesn't know the weight you are carrying so they tend to apply back brakes first or more aggressively than fronts and do thru some magic try to compensate for loads. There could be static averaging or assertive adjustment - again can't say. If it was always equal one would expect back brakes to lock up when no load is aboard too easily which ABS may be doing without much fan fare like a panic stop.
I doubt the master is doing more than static proportioning. If wires go to or near reservoir that would be for fluid level.
Side note: Was thinking of both rear wheel cylinders being frozen or at least the front of two pistons in each but then the pedal would feel ok just not stop properly if frozen in retracted mode via return springs.
Can you even see (be careful doing this if you do) the rear wheel cylinder move with just a helper slightly touching the brake? DON'T ALLOW PISTONS TO FLY OUT AS YOU CAN ONLY SEE THAT WITH DRUM OFF! Competent help required or don't try that.
Air can hide and if still in there back pistons won't move well or much at all hence near no or no braking from back and the air is the soft pedal. Adjustment of rear drum brakes is critical to pedal feel and just where it will begin to brake as noted on brake pedal. If way off, applying parking brake and hitting brakes a few times with a normal feeling pedal would be telling of too much travel in rear drums.
Keep at it. You'll find it,
T
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franknbeans
New User
Apr 14, 2010, 9:38 PM
Post #5 of 6
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Re: 1992 F-250 weak rear brakes
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I FOUND IT! The ABS modulator valve must have a proportioning section to it like you suggested. I took a leap and bought a new ABS valve for about $175 and installed it and rebled the system. Problem solved! Got a good pedal and rear brakes are back to normal. Thanks for all your input!!! Keep up the good work! Your replies were very fast and helpful! THANKS AGAIN!
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Apr 15, 2010, 3:43 AM
Post #6 of 6
(6161 views)
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Re: 1992 F-250 weak rear brakes
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Excellent work! Thanks for enlightening the world (well the site) of the find and fix. Proportioning simply must be done inside that unit by default (and had a fault) if not seen or known of elsewhere,
T
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