Ok, truck seems to stop fine, pedal feedback seems normal but... I was holding the truck and trailer (with the brakes) on a hill. I could feel a very slow give to the pedal under my foot. If I kept applying force, the pedal would slowly drop to the floor after 2 to 3 minutes. The pedal returns to normal if you release it from the floor and apply the brakes again.

My logic tells me that there is a leak somewhere but I searched FTE and saw the following suggestions: master cylinder leak, abs valve problem, air in system, stuck caliper piston etc.

Then I went to the service manual and read the following:

"For example, after braking to a hard stop, the vacuum in a diesel booster may be significantly reduced. If the driver continues to apply the brakes at the stop, the vacuum will start to build again and the pedal will sink as the increasing boost moves the diaphragm forward. This doesn't mean there is a fluid leak or a malfunction in the system. "

Huh? Maybe i don't have a problem after all. What do y'all think?

thanks, dave in Va

94.5 f350 kabinchassy

 

Your master cylinder is shot...replace it before it fails completely...

 

I agree the master cylinder is your problem.

 

I woud also say that it could be the booster. It is behind the mastercylinder. This sympton is very common with a booster going out. Not to mention I had it happen to me once. Let us know what happens

 

Allright, has anyone fit a f450 master cylinder onto an OBS truck, supposedly they have a 1 5/8 piston as compared to the 1 1/8 that is in place on this f350?

 

Not sure on that but keep us posted because I want to know the answer to that one.

 

OK, here's what I've found so far, digging deeper into the service manual:

"PINPOINT TEST H: BRAKE MASTER CYLINDER DIAGNOSIS GUIDE, PEDAL EASES DOWN SLOWLY


H1 VERIFY CONDITION


l Check if condition occurs during actual stopping application by depressing the brake pedal while the vehicle is moving.

l When does the condition occur?

Condition occurs only when vehicle is stationary
No action required. (REFER to «Normal Condition» , under Brake Master Cylinder in this section.)"

also:

"PINPOINT TEST D: POWER BRAKE FUNCTION TEST


D1 BRAKE PEDAL CHECK


l Key OFF.

l Pump the brake several times to eliminate any vacuum from the system.

l Push the pedal down as far as it will go.

l Note the effort required to hold the pedal in this position.

l Does the pedal gradually move downward?

Yes
GO to «C1» .

No
GO to «D2»."

d2 test gives you a normal condition if the pedal moves down as the vacuum builds when you start the engine.


as for the f450 master cylinder info, here's a thread: btw the non superduty master cylinder has an integral proportioning valve and the service manual described the OBS superduty truck (f450) as having an external proportioning valve so that may or not be an issue.

https://www.ford-trucks.com/forums/5...?highlight=450

 

OK that is interesting but I got more from the link. Thanks for the link. BTW How and where did you find a post that is that old?

 

It's pretty normal for the pedal to slowly sink. It has to do with the vacum rebuilding when you push the brakes. Its very common for this era truck. Ford now has what is called a Zero lose brake booster that doesn't do it. There are time when it could be a bad MC or booster or even the ABS valve but this instance sounds like average brake pedal drop.

oh yeah and not sure about the other MC fitting. I think the mounting bolt spacing is different.

 

i have a 90 f 350. when the engine is off i have a firm pedal. one vacuum is applied the pedal goes right to the floor. in a hurry. immediate. if i pump the pedal fast it will firm up for a second and then right to the floor. ideas??

 

If it always goes to the floor...Your master cylinder is shot...replace it before it fails completely...

 

Dent, I have the same problem on my flip truck. I've replaced the rear abs module under the driver floor on the frame. No change.... I bought the F450 master and hogged out the holes so it would bolt to the booster. No change. Pedal still falls. All the brake hardware is fine. All pads and shoes about 1/2 to 3/4. The only thing I haven't replaced is the booster. This problem is driving me crazy!!! I've missed selling the truck because the potential buyer didn't like the brakes. When you apply the brakes, it's windshield city!!! I mean this truck litterally locks down. If you apply them really lightly, they work fine. But if you are holding the brakes going down a steep hill, the brakes fade and the pedal goes down. That same buyer called me back and said it couldn't be fixed because his mechanic told him so.........That alone is the reason I've not advertized this truck. I can fix anything.... And I want to fix this just to prove him wrong. I will replace the booster as soon as I have the chance and let you know. And I will be watching this thread closely.

 

I have this same issue with my 95 cc350. Brakes work great similar to RubberDuck. But when sitting still with engine running if you apply extra force to the brake pedal, it will slowly go to the floorboard. Something is wrong!! I have checked break pads and shoes, checked for leaks and all looks good. Vacuum pump was replaced due to failure yr ago. When this happened it was VERY difficult to stop the truck. My thoughts are if the pedal is going to the floor fluid is going someplace, right...??? If there are no leaks then it is by passing someplace. Can the abs valve do this? Or is the master cylinder by passing? How could a bad booster cause the pedal to go to the floor? Looks like the line pressure would go through the roof causing a wheel cylinder or caliper to blow a seal.

Anyway, I would be very curious to what is going on. I have read several other posts with the same topic but have not seen a good logical fix.

 

Cody I did the same thing with my 95 when I got. I replaced everything except the booster cause I decided to just live with it. My pedal dropped very slowly sitting at a stop. Ford has the Zero lose brake booster. Thats what Cody's truck needs.

If the pedal drops fast or has a problem stopping then you probably need a master cylinder.

you can build a line to bypass the ABS valve. i don't remember the fitting sizes but make a line with the two sizes of fitting used there and use unions to put it back together where the RABS valve was. that will tell you if the ABS valve is the issue.

After working on my 95 I paid attention to every OBS truck that came into the shop. Every single one of them, gas or diesel, did the same slow pedal drop. Thats when I decided to live with it since my truck stopped great.

I will try to find the part number for the zero lose brake booster for you guys.

 

here a link with part number
http://catalog.powerstrokeshop.com/p...ts&searchAll=1