Hello. This is my first posting of a question on this forum. I have a 1994 F250 4X4, 7.3 factory turbo, all stock, with automatic and about 98,000 miles. Many of the topics on this helpful forum are reoccurring and I have read many, many of the brake problem threads, seeking the answer to my problem. I have the seemingly common "low pedal with quick pedal drop after brake application" syndrome. Brakes work well enough, but it won't pass state inspection with this problem. My dad and I installed the 2nd master cylinder yesterday (bench bled, NAPA OEM replacement) and spent the rest of the day bleeding the brakes (RR, LR, RABS, RF, LF), all to no avail. Today we bypassed the RABS valve on the inside frame rail, drivers side, hoping that was where we were bypassing. After more bleeding nothing changed.

I replaced the front right caliper three days ago and the rears were all dealer replaced less than 10,000 miles ago. Also replaced all steel brake lines with the exception of the line going to the right rear which looked fine.

Many say the MC is the culprit (even after 2 of them), but I've seen reference to the rubber lines failing internally, calipers bypassing internally, and RABS valve bypassing. I would like to bounce an idea off of you folks that my dad came up with. I'm talking out loud here so please feel free to correct me... Not wanting to keep replacing parts (aka "if it ain't broke..." or, more likely, I'm just cheap...) until the problem goes away and being fairly certain the brakes didn't perform like this when the truck was new (I bought it with about 75k miles on it), it seems we should be able to isolate the problem. The fact that the pedal drops indicates somewhere something is bypassing, doesn't it? Either that, or the master cylinder cannot generate the volume of fluid fast enough to keep up with one or more of the other components. This would explain why the larger bore master cylinder has solved the problem for some, right? The higher volume it generates is compensating for a problem in another component. So here is what we have decided to try next... cap off each of the four steel lines at the respective caliper/wheel cylinder (or rubber lines leading to them). Once all are capped we can 1) check the master cylinder and then 2) systematically go around the truck connecting only one of the four lines until the problem caliper and/or cylinder is isolated. We can use the caps to bleed the "dead end" lines and the bleeder on each wheel once a line is connected. If our thinking is correct, this could identify, for example, that the left front caliper (which was not replaced) is bypassing internally or the rubber line going to it is failing and either one of these is causing the pedal fade.

What do you think of this? Does it make sense? Any help is surely appreciated!

 

first have you bled them in the proper sequence. not sure what it is but it has one.

 

Thats the right order, start with the brake furthest from the MC (Pass. rear) and work your way to the MC, driver rear, pass front, and end with driver front. I don't know about RABS though.

 

Thanks for the help. I'm on my third big bottle of brake fluid with all the bleeding...

 

Fullcircle,
Welcome to FTE and the IDI diesel forum.


If you have a sinking pedal and no fluid leaks anywhere, you are looking at a bad master cylinder on a non RABS truck.

The mastercylinder is the only place fluid can bypass the piston seals and not show up on the ground somewhere.

I do remember someone posting that was possible with a RABS, but since you have bypassed the RABS, that is eliminated from the system.


I had a problem like this a while back, replacing master cylinders for the same problem after only a few miles driven.

One evening a mechanic friend of mine called with computer problems he wanted me to look at, but I told him I had to change my master cylinder again because of a sinking pedal.
He asked if I had a new master cylinder, yes, so he said if you fix my computer he would fix my brakes. Deal.

So I went over and while I was working on his computer, he fixed my brakes.

When I got the computer fixed he said I have to show you this, he was holding a container with fluid in it.
In the container was brake fluid with a bunch of metal shavings at the bottom.
What he had done was fill the master cylinder with fluid and pumped it all out into this container twice.
Seems the factory is not cleaning the metal from the machining process out of the master cylinders before assembling it.
The metal filings cut the seals after only a few applications of the brakes.
The same master cylinder has been on my truck for three years and counting.

We talked about this, and this is not an uncommon thing with parts manufactured out side of this country.
He also pointed out, if you don't flush the master cylinder out, guess where the filing wind up?
In the wheel cylinders or RABS valve.

Quality control is not what it used to be, but you also have to look at where some parts are made these days.

 

Thank you, Mr. Sponaugle. I saw where you posted your "multiple MC" experience on one of the earlier threads involving this brake problem. My dad and I felt the same way which is why we replaced the MC a few days ago (I only learned of the RABS valve internal bypass possibility the night after replacing the MC, hence the backwards order of doing things). , But then I see other experiences where the two piston calipers were suspect, or that not replacing both calipers at the same time could yield the same result. What are your thoughts on capping the lines as I mentioned? My thinking is that it will ensure the MC is isolated as the only place any bypassing can occur: it will remove the other possiblities (calipers, etc) from the system. If I can isolate the problem to a faulty MC with certainty then I will not have a problem returning the bad one.

Incidentally, what MC did you end up using? I have seen reference to the larger bore solving some of the other folk's problem. I also recall you installed a F450 system that runs off the power steering pump or something, right? I might consider going that route pretty soon.

Thanks again.
-Roy

 

It seems you could test the m\c by plugging the hole at the m/c.

 

JNKDad,
That's true, but if it doesn't fade (meaning it's not the MC), we won't have to re-install the lines and re-introduce air to the system. As I'm typing this, I just thought to ask if anbody can tell me what the fitting that goes in the rear port of the Master Cylinder is? It looks like some kind of valve or something. The brake line goes into the fitting which goes into the MC.

Thanks.

 

Would anybody mind telling me how to edit my signature so I can include my truck info? I can't seem to find where to do it.

Thanks!

 

You can edit your signature after you have posted 25 times. click User CP and edit signature

 

OOH I only Have four more posts to make the change.....

 

I am still using the stock 86 master cylinder.

What I changed was the vacuum brake booster was swapped out for a hydra boost brake booster that runs off the power steering pump.

My luck with vacuum pumps is only slightly better than it was with master cylinders.
At least the vacuum pumps were lasting for about a year before they died.

Using new parts the hydra boost cost a little over 500 dollars, but braking with a load is much better.

Seat of pants performance meter says I can stop easier with 2 tons on the truck now than I could with an empty truck before the upgrade.

 

Dave, When the vac pump dies does it give the same signs as a bad vac brake booster. I.E. int. hard brake pedel??

 

Yes, another sign is the heater will default to the defrost position.

 

Fullcircle try this: Pump up the brakes quickly and hold it down.....do they stay up.. If they sink bad on normal application the MC is bad. Pumping fast and holding spreads bad seals on the piston and the MC will operate....... normal application they will not spread or as Dave said be full of crud..... incidently that is how Ford checks for a bad MC.