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Discussion Starter #1
on my 95 PSD. this is the third master cylinder that I have put on the truck (new, not reman or rebuilt) and the problem has not changed. about 5000 miles ago I put on a new brake master cylinder and the problem did not go away.... now after installing drop-down brackets, and front brake pads, I thought I would replace the master cylinder again on the off chance the last one was faulty. the brake pedal still slowly goes to the floor. it is drivable and stops great, but at a stop light I need to let up and step on the brake pedal a couple of times to keep stationary (or put it in park). where could the pressure be going? could the proportioning valve be leaking by? where is the proportioning valve located? there is no external leaks in the system but there must be an internal leak for it to act like it does. as far as I know the proportioning valve is the only other thing in the system that could have and internal bypass, and I'm not sure it could cause this problem. thanks Lou
 

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there isn't a line that has rusted thru is there and is just leaking fluid cause i just had my truck do that
 

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How did you bleed the system? (IMHO the best way to bleed is a power system that connects to the master cylinder and pushes fluid through; the second best is the vacuum pump at the bleeders; the worst way is the old "get somebody to pump the brakes while you bleed it" approach). There is also a bleeder at the proportioning valve, which is on inside of the drivers frame rail just down from the master cylinder.

Some folks must've complained about a very-slow creep downward with the stock vacuum booster, apparently. This info is from the Dieselmann site, maybe this will help:

"1997 and earlier diesel trucks equiped with vacuum power brakes--Low/creeping pedal:
Many owners have complained of a low or creeping brake pedal on their trucks, often mistaken for a bad master cylinder or Rear ABS valve. If the condition occurs after the vehicle has come to a stop, and no other brake concerns are found, (worn or out-of-adjusted brakes, ****ed pads, or hydraulic problems) the condition is normal and the result of the vacuum pump replenishing the vacuum inside the power booster. Ford has developed a Zero-Loss Travel brake booster to correct this condition if the customer finds it unacceptable. This part is only for use on diesel-equiped trucks--P/N F5TZ-2005-CA--and is not a service replacement; it must be ordered by the part number, not vehicle application.

This condition is similar to the low brake pedal concern on all 1995 F-series that required a larger-diameter master cylinder:

Trucks without cruise control:


F6TZ-2140-AB
F-250 Gas 4X2; F-350 DRW, SRW Gas 4X2, SRW 7.3 4X2 168" WB
F6TZ-2140-EB
F-250 4X4, 7.3 4X2; F-350 SRW 4X4, SRW 7.3 4X2 133" WB
Trucks with cruise control:


F6TZ-2140-BA
F-250 Gas 4X2; F-350 DRW, SRW Gas 4X2, SRW 7.3 4X2 168" WB
F6TZ-2140-FB
F-250 4X4, 7.3 4X2; F-350 SRW 4X4, SRW 7.3 4X2 133" WB
These part numbers are from TSB's 96-25-19 and 98-5A-34, and are not normal service-stock replacment parts."
 

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Discussion Starter #4
there isn't a line that has rusted thru is there and is just leaking fluid cause i just had my truck do that
if there was a rusted thru brake line, I would think I would have to add fluid every so often and that I would see fluid seeping or leaking. when I step on the brake the pedal will bottom out on the floor in about 45 seconds. the fluid level in the master cylinder never changes.
 

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Discussion Starter #5 (Edited)
Some folks must've complained about a very-slow creep downward with the stock vacuum booster, apparently. This info is from the Dieselmann site, maybe this will help:

"1997 and earlier diesel trucks equiped with vacuum power brakes--Low/creeping pedal:
Many owners have complained of a low or creeping brake pedal on their trucks, often mistaken for a bad master cylinder or Rear ABS valve. If the condition occurs after the vehicle has come to a stop, and no other brake concerns are found, (worn or out-of-adjusted brakes, ****ed pads, or hydraulic problems) the condition is normal and the result of the vacuum pump replenishing the vacuum inside the power booster. Ford has developed a Zero-Loss Travel brake booster to correct this condition if the customer finds it unacceptable. This part is only for use on diesel-equiped trucks--P/N F5TZ-2005-CA--and is not a service replacement; it must be ordered by the part number, not vehicle application.

This condition is similar to the low brake pedal concern on all 1995 F-series that required a larger-diameter master cylinder:

Trucks with cruise control:

F6TZ-2140-BA
F-250 Gas 4X2; F-350 DRW, SRW Gas 4X2, SRW 7.3 4X2 168" WB
F6TZ-2140-FB
F-250 4X4, 7.3 4X2; F-350 SRW 4X4, SRW 7.3 4X2 133" WB
These part numbers are from TSB's 96-25-19 and 98-5A-34, and are not normal service-stock replacment parts."
Has anyone bought and installed one of these? I dont understand exactly how the vacuum brake booster could cause the pedel to creep. I thought the brake rod from the pedal to the master cylinder was a direct mechanical connection, with a vacuum diaphram assist as it passed thru the vacuum chamber. if this is the case, then the master cylinder would have to be loseing pressure for it to bottom out.

if the brake rod is not a straight thru to the master cylinder and the connection is only made with a vacuum diaphram, then how could the system be fail safe in case the engine stalls or the vacuum pump fails?

pedal creep is about 45 seconds from brake applied to pedal bottomed out on the floor. I will unhook the vacuum line and see what happens then... does anyone know of any troubleshooting proceedures for this problem, rather than just replacing parts... like I have been doing. thanks for any help... Lou

this has been edited with additional information....... when the truck is not running, there is no pedal creep. with the engine running and the vacuum pump line unplugged, there is no pedal creep. with the engine running and the vacuum line hooked up as normal, there IS pedal creep.

there is a good supply of vacuum from the pump. with the engine running and the vacuum line hooked up, the pump is not pumping. if I step on the brake pedal and then release it, the pump runs until it replenishes the vacuum in the chamber... BUT, also while the pedal is held down the pump is pumping air from the chamber, steadily. it would seem like the diaphram is ruptured, although I am still unsure why the pedal would creep to the floor, rather than just not developing the vacuum necessary to create the full braking assist as designed.

the vacuum line is in the front of the chamber and the master cylinder is located there also.... should there be a gasket between the chamber and the master cylinder? mine does not have and never had one. could this be a vacuum leak and causing a problem? still looking for the answer. thanks Lou
 

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I don't know the whole relationship, as to why some trucks lose pedal when the vacuum pump is going bad, that TSB seems to spell it out. I had the same problem as you on my trucks, pedal would eventually fall to the floor, while the truck was running, sitting at a light, or whatnot. I heard people say they had a super hard pedal, when the vacuum pump failed, but that wasn't my experience.

I changed out the vacuum pump when this happened to me, and it's been normal for a few years now.
 

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I put in a zero-loss brake booster (the part number mentioned) to solve my problem. Mine was "sucking the pedal to the floor" when I applied the brakes. My pedal does not creep to boot. Cheers!
 

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My 95 PSD is doing the exact same thing right now. (158K) Everything has been replaced. (master, vacuum pump, and Antiskid valve) I'm going to install the zero loss boster and hopefully that will do the trick.
Is the zero loss booster going for $221 everywhere in the country or are they sticking it to us in California?
 

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My 95 PSD is doing the exact same thing right now. (158K) Everything has been replaced. (master, vacuum pump, and Antiskid valve) I'm going to install the zero loss boster and hopefully that will do the trick.
Is the zero loss booster going for $221 everywhere in the country or are they sticking it to us in California?
Go to AutoZone.com | Get In The Zone! and search the part number. Mine was $64 plus $20 core charge.
 

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go to powerstrokeshop.com
 

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I'll second Powerstroke.com (remember oerder by part number not application). Cheers!
 

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Hi Folks,
I'm having the exact same problem with my 1998 E-350 PSD cut away van. Changed everything except ABS Modulator and Vac Booster. I went up to Powerstroke.com and they don't list it for my 1998. Can I use this same O loss model Vacuum Booster on my rig.

Thanks guy!
 
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