2000 F350 7.3 liter diesel, 4 wheel drive, 4 wheel ABS. Replaced all 4 calipers and pads, new master cylinder. All was well for two weeks, now pedal occasionally goes to floor while braking with limited stopping power. Sometimes pedal is hard and they work fine. Never know what to expect. It seems stabbing at the brake pedal will often restore them temporarily. Also, pedal always goes to floor without engine running. No ABS light. Can anybody shed any light??????????????

 

I have copied the following text out of a Ford workshop manual. My guess is air in the system, or master cylinder needs replacing.

 

Thanks for the info....I will try to bleed system again in the morning.

 

Classic situation for a cup in the M/C.

Since the back of the M/C bore never gets used some debris and oxidation usually builds up there. After bleeding the brakes for whatever reason, in this case new calipers, the piston cups get down into this region and get torn or the edges abraded. Then you start to loose one of the two circuits of the M/C and only have 1/2 brakes. Since the ABS is not involved there would not be any ABS or rake light illuminated on the dash.

A hard hit or second quick apply often gets the seal temporarily to seal.

 

But the Master cylinder was also replaced as par of the brake job. Could the new one have failed after 2 weeks????
The fact the pedal goes to the floor with engine off must be significant. Then brakes work occasionally with engine running.

 

Was it really a new MC, or a rebuilt?

 

Bobby,

Sorry I was going back and forth between three different forums and didn't pick up that you had replaced the M/C. But a new master can still be an issue, it happens.

The 4WABS can have a bleed down issue, but it is rare. To check it you have pull the battery and get down to the ABS controller. But the one instance I came across that in 10 years of SD's it was a slow bleed down, not the rapid push all the way to just off the floor.

Air in the system won't act like this. It has to be a mechanical seal failure.

When you installed the new M/C did you bench bleed it first or do anything to try to get the piston bore filled with fluid?

 

Jack,
Thanks for the reply. I have been told you are "the brake Guru" and appreciate your advice as I am baffled at this point. Yes, I did bench bleed the MC before installing. I plan to install plugs and test today even though it was new. I initially did a gravity bled while working, then did a manual bleed afterwards, starting at right rear, etc.

After reading some posts...I have to admit, I added some power steering fluid rather than Mercon to my power steering pump. It was a small amount, but I will also flush it according to instructions on this site. I can't imagine this is the problem.....your thoughts???

Jack....the fact that the brakes "ALWAYS" go to the floor with engine off and are hard at least initially with engine running, must be a significant clue, just can't figure out what it means!!!

You mentioned the ABS controller under the Battery. Is there a way to test this and do the symptoms point to it??

Also, other posts mention the hydro boost. I am assuming this refers to the power steering pump, correct?? The power steering works fine and makes no noise.

Any suggestion would be greatly appreciated. I need the truck!!

Thanks again,
BobbyD

 

Bobby,

God I hate that term from over there. I’m from the engineering side. While I can teach mechanics a few things, they teach me as well.

The reason I was asking about the bleeding and prep was I’ve seen where new M/Cs are installed dry and bled on the vehicle developing issues quickly. I believe in those cases the bore is slightly dry while being stroked and the cups get damaged. Sounds like you whetted yours out though.

Adding the PS fluid would not have hurt anything. It’s a generic fluid.

As I said it would be rare if your ABS controller would be having a bleed down. But it is cheaper to check it then throwing new M/C or Hydroboost to try to solve the problem. If it is bad you’ll have wished it wasn’t due to the price. I notice you have a 2000MY and I don’t remember if with the 7.3L the battery has to go or not. Look around for the ABS controller and see what is in the way. Anyway, here is the check.





One thing to note on the bottom of the page is it gives you a good clue about how to tell if a master is bad.

One way that many people suggest to test a M/C is to put plugs in each port and see if the pedal goes down. This is not always reliable. First, you have to be able to get the plugs. But trapping the ports does not let the piston stroke very far, and sometimes it’s not enough travel to get a cup to roll over. It makes you think everything is fine internally.

Hydroboosts are the booster between the pedal and master cylinder, powered by hydraulic pressure from the power steering pump rather then the typical gas motor vacuum booster. In laymen’s terms, they are more like a control valve for hydraulic boost then the assist given by vacuum boosters. They do not operate with a linear feel or actuation to them.

With a vacuum booster, you can shut off the motor, exhaust the vacuum reserve and feel the direct manual force to the M/C with a hard but linear operated pedal.

Hydroboost is more like a slush box; the linear feel obscured by the mainspring and valves in the unit. It’s like pressing on a rubber ball, but when running the hydraulic boost brings back some of the linearity. If you do a panic stop, you push right through that “hard” feel and are back to the rubber ball situation.

There is no good quick hard test I have for you to identify if the hydroboost is bad. And the vague feel of a hydroboost noted above makes on-line diagnosis sometimes difficult. People can drive a hydroboost for years under normal circumstances and never get the opportunity to have the rubber ball effect, then when you are trying to diagnose something like this is clouds the situation.

Here is one check I can give you, using the highly technical equipment in the brake testing industry that we call a tape measure. If you hook the end of the tape measure to the back of the brake pedal and run it through the steering wheel so you can take readings at the bottom of the wheel, engine off hard pressure should have a travel of about 3 inches. Engine running should give you about 4 inches. Hard pressure isn’t trying to push through the back of the driver seat, but about 50lbs of foot force. If you have more then that, yeah, you’ve got problems.

...the fact that the brakes "ALWAYS" go to the floor with engine off and are hard at least initially with engine running, must be a significant clue, just can't figure out what it means!!!

That still brings me back to the M/C, and the clue in the Ford service manual I pointed to. M/C cups usually leak under low pressure, which would be normal light stopping and especially when the engine if off without boost. Quick, hard applications usually build enough brake fluid pressure to expand the cups forcefully so they seal. After you check the ABS controller I would use the light pressure / hard pressure check looking to the M/C. That’s where my best guess is.

Keep in mind that (my prices) from Ford the M/C runs about $140 and the Hydroboost about $320.

 

It's quite strange that the pedal goes to the floor with the engine off.

The diesel uses a Hydroboost system, so it needs power from the engine (power steering pump) to create hydraulic pressure to assist in braking.


First question:

Why did you replace all 4 calipers and master cylinder in the first place? Was it doing this BEFORE all the work?

 

Hi Jack,

First let me thank you for all your help & patience. I wanted to let you know that ....just as you suspected it was indeed a failure of the master cylinder. I think deep down, I new you were correct, but my brain wouldn't let me go there, simply because I had replaced it not 2 weeks ago. Sometimes I have a habit of gravitating to the worst case scenario first. Perhaps sometimes you need an outsider to state the obvious. Anyway...."kudos to you" and for a "Curmudgeon" you strike me as a pretty good guy.

Thanks again

BobbyD

 

Glad I could help Bobby. You had the classic symptoms of a M/C. ABS controller too, but that is so rare. I figured I give you everything to chew on so you could work you way through it. No one likes to spend money and I hope you got the second M/C on warranty.

I'm on four "Diesel" forums, the least here. The "Curmudgeon" is self described for two reasons; like Mom I don't have the 'tactfulness" gene, and I usually don't have a lot of patience.

But glad you got it worked out and can now stop safe.

 

Hey Jack,
Wish I could Have talked to you $1000 ago I had the same problem as Bobby with the brakes stopping then fading on to the floor after the truck stops, and the truck will creep off if I dont pump and push the pedal, I have replaced rotors pads ft & rr calipers in front, bled and installed 2 M/Cs installed an ABS unit 2nite didnt get it finished thou. But my truck had the same problem as Bobby seems to me like. My truck sometimes pulled to the right. Certfied Ford Diesel mech told me it was the ABS, I argued with him about it, bcause I felt I had pressure bleeding by something, then yesterday my brakes failed and would not stop the truck pedal stopped about an in. off the floor, so I decided I would try it. is there any special way I need to bleed the ABS unit or will it be fine to do it the usual way by just bleeding the brake system.
Thx Glenn

 

To bleed the ABS controller you really should have the Ford IDS or any unit that can operate the ABS controller pump and valves. This is a hard system to get all the air out of.

Now that I've said that, we used to get prototype ABS units we had to install in vehicles and not have any factory scanners to work with. It was a bear and could take an entire day, but we would get it done. So here's the backyard approach.

First you do have to bleed the the entire system just like a car of the 60's. While you are doing that, crack the fittings at the ABS controller to try to bleed out as much air as you can. Sometimes tapping the controller lightly with a small hammer helps to break loose any air bubbles attached to walls.

Next we had the luxury of having a full lift to place the vehicle in the air and use "air driving" to activate the ABS. You may not have that luxury. I would put the rear axle up on jack stands so you can put the vehicle in drive and get the rear axle spinning up over 25 mph. Apply the brakes a few times lightly which will kick off the ABS function. Without the front wheels moving, the ABS software will throw the ABS warning light due to differences in front wheel to rear wheel speeds. That's OK and will correct itself later.

After several activations, re-bleed and see how the system is. Repeat as necessary. Not sure if it's feasible for you but after we got an OK pedal we would take the vehicle out on the dirt road next to our building and fire off the ABS at 40 mph, then come back in and bleed again.

We might play with this all day until we got a pedal that we felt was high and hard. We still might re-bleed the vehicle after putting the vehicle out on the road burnishing the brake for 200 miles just to make sure.

On production vehicles where on of the driver-mechanics had a brain fade and let the vehicle brake hydraulics get air into the controller, we would definitely go to the IDS and use the systems functions. It is the beat way.

Going back to your and Bobby's original problem. A pedal drop with absolutely no external fluid leak is an internal bypass. There are only two locations that that can occur in - the master cylinder and the ABS controller, 4WABS or RABS. The ABS check is time consuming but it's cheap compared to throwing parts. The fast apply hold / slow apply leak down check
for the M/C is also good, but not foolproof.

But over the years on many different vehicle platforms and manufacturers forums I've seen a lot of cases where replacement master cylinders end up failing. Not sure if it's the quality of the aftermarket or the methodology of bleeding in the new master, but I've seen it more times that it surprises me.

All I can say to people is if you get a replacement master cylinder, make sure you keep the receipt in case it goes bad in a few weeks.

 

Hey Jack,
Thanks so much for the info, aquired a used ABS unit installed it today and guess what after bleeding and bleeding the pedal was firm on stop not spongy but then it happened the same dang thing now 1100.00 later the truck stop then the pedal started fading on to the floor and the truck started to creep off I had to press the pedal on down with maybe 1 1/2" off the floor travel left. I am totally discouraged and I gonna head to Ford dealer in the am for a new M/C and start over. Is it possible to bench test the ABS module or do I need to just head to dealer with 1400.00 for a whole unit. Please help. The 2 M/Cs I've used were from Advanced Auto Parts. Could please explain the fast / slow deal with M/C test. I think I'm gonna put my old ABS unit back on when I put the new Ford M/C on. Where did u get the link info on the HCU back up the thread to Bobby? What ur thoughts. ThX
Glenn