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sinking brake pedal, F350 diesel


stoneloco808
02-12-2006, 09:20 PM
1995 Ford F350 4x4, 7.3 Liter Diesel, A/T, A/C

Okay I hope someone could help me out on this problem, the brake pedal gradually sinks to the floor while the truck is running while stepping/holding down the brake pedal, then the red brake light comes on in the instrument cluster. When the trucks engine is not running I could pump the pedal to clear the vaccuum booster and hold the brake pedal, it stays steady.

So far I have replaced the vaccuum pump and brake booster. I also readjusted the rear brakes. The truck stops fine, but the sinking brake pedal just doesnt sound safe. Someone please help me out......

unclebubbles
02-13-2006, 12:23 AM
If you dont have a leak somewhere thats causing you to have to add fluid, then its the master cylinder. The piston seals are leaking allowing fluid to bypass internally. If you look under the dash at the firewall where the rod goes into the master cyl. , its probably leaking some there. Good luck.
Later
Ed

stoneloco808
02-15-2006, 02:59 AM
When I replaced the vacuum brake booster, the rear of the brake master cylinder was dry. Like I mentioned before, when the engine is off and the brake pedal pumped to the point of no vacuum in the booster. I could stand on the brake pedal and the pedal will not fade. The brake pedal sinks a bit when the engine is running. I know the brake pedal will sink/drop a bit when the engine is running. But in this truck it will sink/drop a bit more than any other truck and the red brake light in the instrument cluster will light up as well. But when I let off the brake pedal the red brake light on the dash turns off.

wafrederick
02-16-2006, 08:11 AM
You problally have a rusted out brake line somewhere that is leaking.Is there a spot on the floor?

ModMech
02-16-2006, 08:58 AM
1995 Ford F350 4x4, 7.3 Liter Diesel, A/T, A/C

Okay I hope someone could help me out on this problem, the brake pedal gradually sinks to the floor while the truck is running while stepping/holding down the brake pedal, then the red brake light comes on in the instrument cluster. When the trucks engine is not running I could pump the pedal to clear the vaccuum booster and hold the brake pedal, it stays steady.

So far I have replaced the vaccuum pump and brake booster. I also readjusted the rear brakes. The truck stops fine, but the sinking brake pedal just doesnt sound safe. Someone please help me out......

Unfortunately, this is completely NORMAL.

If you have the Factory Service Manual, you can read for yourself, but according to FMC, this condition is "normal".

I have run into this dozens of times myself, and trying to convince the customer of this is not an easy task, eventhough it is printed in black and white.

The reasons are pretty obvious, when you really think about it.

In a "normal" vacuum assisted situation, you have the maximum vacuum available during braking with the throttle plates closed on a gasoline engine. Well these (and most) diesels do not have any throttle and cannot make engine vacuum at all.

When you depress the brake pedal, a small valve is opened allowing stored vacuum to be vented to a diaphram. Doing this lowers the vacuum in the storage "tank" that is part of the booster assembly. In a gasoline engine, there is so much vacuum pressure and volume available during closed throttle deceleration, that you never even notice any vacuum was "used" as it is replentished immedaitely by the engine.

Diesel engines installed in vehicles with vacuum assisted brakes all require some form of vacuum pump to be fitted. The most common way is to drive them off the engine belts where the amount of vacuum is related to speed and time (it takes some time to produce or replentish used vacuum).

So, when you have a truck like yours with an engine driven vacuum pump, and you are aproaching a stop, you remove your Rt foot from the accelerator (GOOD thing), the engine speed drops (reducing the vacuum pump capacity), and you depress the brake pedal (which causes the vacuum in the booster to drop). As you are slowing down, the vacuum pump continues to run and replentish the used vacuum, THIS causes the pedal to "sink" because as vacuum is replentished there is ever-more "assist" available to the brake system.

The brake booster is basically a canister with a diaphram inside that is sealed on one side, and vented to the atmosphere on the other (brake pedal) side. The sealed side is placed under vecuum by the engine or vacuum pump in this case. The assist available (in PSI) is the area of the diaphram times the pressure difference between the two sides ofthe diaphram (A*Pd). Since the area does not change, but the Pressure does (and does so MUCH more in a system with a vacuum pump) you will have more assist as the engine driven pump replentishes the used vacuum.

Per the FSM, if the vehicle stops "normally" DURING braking, and the pedal does NOT SINK DURING braking, but ONLY while sitting stopped, there is not "problem".

unclebubbles
02-16-2006, 03:22 PM
I agree with part of that, and the theory of operation is correct. But, he said the pedal was going to the floor, and the warning light was coming on! This, is not normal whether its gas, diesel, running , or parked not running. The warning light is coming on because of hyperextension of the master cyl. , and pressure imbalance. I still betcha he`s either got a leak, or the master cyl. is bypassing. You cant put as much pressure on the pedal with the engine not running and no vacum, as the booster does during assist.
Later
Ed

cujafish
02-16-2006, 06:49 PM
this happens when air gets into the brake lines
if the pistons on the master cylinder are screwed, then it will allow air in
keep filling the master cylinder with break fluid and pump the breaks hard
and let the air out through a connection in the brake line
make sure you get the brake pads close to the rotor while bleeding the air then tighten brake line up or you have to much play in brakes and this problem can start again.

hope yhis helps

ModMech
02-16-2006, 08:01 PM
I based my diagnostics on everything he posted, the litmus tests were:

A) "When the trucks engine is not running I could pump the pedal to clear the vaccuum booster and hold the brake pedal, it stays steady."

and

B) "The truck stops fine"

A: Shows there is NO hydraulic leak, or the pedal would sink even with all the vacuum depleted.

B: Indicates that the RWABS is functional and not leaking, and that the rear brakes are in fact, properly adjusted.

The light comming on is an indication of a problem, or a hyper-sensative vacuum switch. The BRAKE warnig light will illuminate for either a Hydraulic OR a Vacuum issue. If vacuum, it is very common for the low vacuum switch to be bad.

unclebubbles
02-16-2006, 09:05 PM
I based my diagnostics on everything he posted, the litmus tests were:

A) "When the trucks engine is not running I could pump the pedal to clear the vaccuum booster and hold the brake pedal, it stays steady."

and

B) "The truck stops fine"

A: Shows there is NO hydraulic leak, or the pedal would sink even with all the vacuum depleted.

B: Indicates that the RWABS is functional and not leaking, and that the rear brakes are in fact, properly adjusted.

The light comming on is an indication of a problem, or a hyper-sensative vacuum switch. The BRAKE warnig light will illuminate for either a Hydraulic OR a Vacuum issue. If vacuum, it is very common for the low vacuum switch to be bad.
lol Ok, but you left out the most important fact in your litmus test. The brake pedal sinks all the way to the floor, and if there are no leaks, the most logical explanation is the master cyl. is bypassing fluid internally. I just finished a chevy truck that was doing the same thing. With the engine off and no vacum, it would hold a pedal for 30 minutes if you wanted to sit there and hold it, and was sinking to the floor so slowly, it was barely noticeable. But the additional pressure on the master cyl. from the booster with the engine running caused it to eventually go to the floor. The truck stopped fine and although it was low on fluid. it didnt get low enough to get air in the system. When i found the leak, it was a cracked line at a fitting. It`ll be interesting to find out what it is though, could be something else entirely, and both of us are wrong.
Later
Ed

LukeJ
04-25-2006, 12:04 PM
primary cup on the secondary piston in the master cylinder.... replace

Evsman
05-18-2006, 03:05 PM
I have the Exact same problem with my 1990 Ford F-350 diesel! It does just as the first guy stated. Yesterday I replaced the Master cylinder thinking it would be the cure for sure. And It made no difference what so ever. The brake pedal still goes clear to the floor when the engine is running. Yet seems to hold good when the engine is shut off.
I don't get it.

ModMech
05-19-2006, 08:35 AM
If it sinks ONLY while sitting and STOPPED, then it is "normal" (yes, I KNOW it should not do this, but they often do).

If, it sinks WHILE STOPPING, then you have a problem. Since you already replaced the MC, the next most common part is the RWABS valve leaking down.

95F350
02-09-2013, 03:24 PM
I've been there with this problem. You need a Zero Loss Travel brake booster to correct this issue. I searched for an aftermarket unit to avoid patronizing the Ford parts counter- it is their production of this POS that forced replacement (wouldn't pass a safety inspection)- but I couldn't find one.
If there was a leak in a brake line you would probably see a spot on your garage floor and after awhile you would find the fluid low. The only other problem I can think of is the master internally bypassing but someone already suggested that. Replacing the booster with the same P/N will give you the same issue. The Ford TSB states the proper P/N for your application, if you go for it they will take quite a bite out of your wallet, the least they can do for you is lookup the TSB for you.

DeltaP
02-10-2013, 09:58 AM
Good info and welcome to the forum. But take note this thread is almost 7 years old.

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