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Junior Member
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Discussion Starter #1
OK, I have a 89 ford f250 7.3l diesel,when i bleed out the brakes,im showing no air at all wheel cylinders,no air at the master cylinder.all wheel cylinders are new,all new brakelines and flex joint lines,new master cylinder,new brakebooster it's all been replaced,brake pedal standing firm,OK,here's what i can't figure out, when i start the truck and hit the brakes the pedal will fade off,sounds like air in lines right,I PROMISE THERE'S NO AIR IN THE LINES,can't figure it out,any one had this problem or got a suggestion,any help appreciated,also i heard something about changing master cyl. with one from a differen't year,because it had more flow or something, anyone heard of this,need some input guys
 

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Discussion Starter #2
MORE INFO ON USING DIFFERENT MC.

I found out the other mc. that can be used is off of a 87-94 f250 super duty,they have the hydro boost,this mc. is supposed to have a larger bore and that is supposed to take care of the brake fade in the pedal.I have yet to make t:thinkinghe change, but i will update and post if it solves my problem.The only change i'm supposed to have to make, is to enlarge the mc. to booster holes in the mc.I sure hope this works,the truck has always had soft feeling brakes,everyone says it's normal for these trucks,i hope to improve on that,will keep everyone posted on results.
 

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had this issue many times, If you went to NAPA they should have offered you a '96 modle year master cylinder if you have a low pedal issue.

all you need is a master cylinder for a '96/97 F350 and you will be all set.
 
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What do you mean by "fade off"? I think you also have a ABS unit under the drivers side on the frame. Was it bleed out also? If you are talking about the pedal dropping when applied you may have a bad vacuum pump or loose belt. Tee a guage into the system to check.
 

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Discussion Starter #5 (Edited)
ABS BYPASSED

ABS has long since been bypassed,because i had decided there may be a problem with it.What i'm saying is ,when brakes are applied it stops fine,but then it will fade to the floor,it has always been this way no matter what,not a big deal maybe,but it bugs the heck out of me,because i've never owned a truck that did this.I'm gonna try the different master cylinder,from what i've come up with it has a 1 3/16 bore versus a 1 1/16 bore in my present one.Anyway i'll post the results,could be a while before i get to it. Vacuum pump is pulling 18 psi.,not a problem.
 

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If you have no leaks in the system that would then be a bad master cylinder. One way to verify would be to remove and plug the lines at the master cylinder and apply your "brakes".
 

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Discussion Starter #7
UPDATE TO SINKING PEDAL

:rockProblem solved,used master cylinder off,87-97 f super duty,pedal is firm and holds, doesn't slowly sink.I have some part#,hope they help someone else,RAYBESTOS MC 39876 from napa,orCARDONE 10-4015 from advance auto.This info needs to be easier to find,would have saved a lot of heartache and time.Ford does have a tsb on it, but the auto parts stores didn't know about the upgrade.Just so maybe some guys will understand what i'm talking about,the upgrade has a 1 1/4" bore versus the1 1/16 to 1 1/8 bore in the ones shown for these trucks,so the upgrade provides more flow therefore more holding.Any way you still have to make sure your brakes are properly bled,this only fixes a firm but sinking brake pedal. BUT DAMN IT'S NICE TO HAVE A SOLID PEDAL WHEN YOUR PULLING A 25' GOOSENECK WITH 15 6X6 ROUND BALES ON IT.I'll quit rambling now, always get excited when i finally get something like i want it on my trucks.
 

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If the pedal was dropping it was bypassing inside and was defective. If the pedal effort is good with the larger bore life is good but some times the effort goes up past comfortable when the bore size is increased leaving some without the size or weight to be able to lock up the wheels.
 

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Brake pedal "drop off" is a known and common complaint. If you reference the Ford Factory Service Literature it will describe this condition and there are two things to know:

1) If the pedal "fades" or "drops off" WHILE STOPPING, you have a SERIOUS PROBLEM and need to get it fixed ASAP.

2) If the "drop off" occurs ONLY while sitting (stopped at a light for example), there is nothing wrong at all.

The vacuum pump is building vacuum in the booster as the engine idles, the same pressure by your foot will make more force on the MC as the vacuum increases causing the brake pedal to "sink", often even in the abscense of any defect.
 

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There should be a full vacuum in the booster can in normal operation so building vacuum with the pump should have no effect, the sinking pedal may show a vacuum leak in the booster.

If you have pounded the brakes and dumped the vacuum then it would build vacuum after stopping..
 

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Nope, full vacuum is only built at full throttle, there is almost no reserve vacuum on a diesel with a vacuum pump. The second you let off of the accelerator pedal and tough the brakes, a significant amount of the vacuum reserve is depleted and as the engine idles the vacuum pump continues pulling more vacuum which - given the same amount of foot pressure on the brake pedal, will give you more effective total boost causing the sinking pedal.

A gasonline engine makes most vacuum with the throttle closed, WHILE you are braking, a diesel makes the least under the same conditions.
 

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Nope, full vacuum is only built at full throttle, there is almost no reserve vacuum on a diesel with a vacuum pump. The second you let off of the accelerator pedal and tough the brakes, a significant amount of the vacuum reserve is depleted and as the engine idles the vacuum pump continues pulling more vacuum which - given the same amount of foot pressure on the brake pedal, will give you more effective total boost causing the sinking pedal.

A gasonline engine makes most vacuum with the throttle closed, WHILE you are braking, a diesel makes the least under the same conditions.
are you talking about intake manifold vacuum or the belt driven vacuum pump? If there is no or low vacuum maybe the pump is bad. I had a vacuum gauge on my '97 F350 PS for the vac/hydraulic trailer brakes and it would stay at 27 in/mg most of the time you could make it fall buy pumping the brake fast with the trailer brakes hooked up.

I like the Ben Franklin quote, you must be a fellow Ron Paul supporter.
 

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Yes, our MAN Ron.

Diesels have zero intake vacuum with a few rare exceptions and some operate under positive pressure (2 stroke DDAs and some turbocharged engines), if you have vacuum powered "accessories" and a diesel, you need some form of vacuum pump. The Ford P/Us use a belt driven pump with vacuum brake boosters and an electric pump if you have the Hydroboost system.
 
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