I am having a problem with my brake pedal falling to the floor. I have replaced the original with 2 different new ones but same problem. I have plugged the outlet lines so the fluid cannot get out of the master cylinder. I bench bled both master cylinders by running lines from the outlets back into the cylinder making sure the ends are below the fluid surface. I cannot figure out how the pedal can fall unless there is fluid moving past the piston, but that would mean two new master cylinders bad. The only other possibility is air but this is a problem that first started with the original and with the two new ones. The pedal falls whether the truck is running or not, just does it a lot slower when there is no vaccum assist.

Any ideas greatly appreciated, thank you.

 

Check wheel cylinders, (Leaking at the insides of the wheels, don't have to jack it up...) Bad spots in the lines causing leaks (rust where brackets hold lines to frame.)
I blew a hole in the metal brake line while parked, and didn't know it till I dropped my lighter and bent down to pick it up, and seen the drip.

 

Speaking from experience, stranger things have happened. Try a different brand if you can.

 

Can you describe a little better exactly what is happening. Is it that you have a good brake but if you keep a constant pressure on the pedal it will slowly go to the floor? Or is it that the pedal goes to the floor with no force on it? If it is the first one you may have a defective RABS valve body.

 

HrdDrv, no leaks, I have the outlet ports on the master cylinder plugged so fluid is not leaving the master cylinder.

Beastly, I might try that but I am also thinking about finding a working used one to try to see what happens. I have replaced just about everthing, the master cylinder, booster, RABS valve, everything new in the rear brakes, except e-brake cables which I probably should of done.

Lazy K, I am almost positive it is not the RABS because I replaced that and have now bypassed it to take it out of the game.

As it is right now, the ports are blocked off on the master so no fluid can leave it. The truck is a 96 f250 diesel. With the truck runing, put foot on brake at first feel ok but if keep pressure, light pressure, the pedal continues to go all the way to the floor. With truck off and push pedal, feels hard but if I keep pressure on it, continues to fall to the floor, just takes a lot longer. Also when the brake system was hooked up with the first new master and system bled, it take a lot more pedal force to stop the truck. Before the problem it only took light force to stop the truck, now when you first apply the brakes it is easy but must keep pushing and increasing the pressure to get it to stop.

Like I stated in the beginning, I bench bled it according the directions but my friend at the parts store says there is a new procedure where you keeps the ports blocked and continue to pump it until you can anymore, if I remember correctly.

Problem first started when I was coming home lightly braking down a hill, the pedal felt soft all of the sudden and the brake light came on. With no leaks visible and the fluid in the cylinder not dropping, I though master cylinder. Changed it, same problem, so I have changed the booster, RABS and rear brakes, and now on second master same problem. It is getting kind of ridiculous and expensive now.

 

Ridiculous? You bet. Expensive? You bet, with what you're doing. Stop throwing parts on it and pray!

Diagnose it!

Pedal goes to floor without resistance:

Frozen calipers, wheel cylinder
Contaminated shoe and/or drum
Hydraulic brake hose(s) caved in
Bad booster
Differential valve

 

It not as easy as you think. At first I wasn't too familar with some of the testing techniques but also with the master, it seems like if I push hard and fast it acts like it hydro locks to give the appearance that it is good and if you don't wait long enough you won't notice it.

And I don't know if you read everything but I have the outlet ports plugged so the only parts of the brake system that are working are the master and booster.

And I don't like to just throw money and parts at stuff, I want to know what is the problem, but if you sometimes don't catch the symptom in a diagnostic test then sometimes it will lead to this.

 

I know, that was generic. I didn't realize the diesel had the hydroboost setup. I know nothing about the hydroboost.

But why do you have the outlets plugged? Did you bleed the brakes like a madman after hooking it up on the previous two M/Cs? How does the brake fluid look? How old is it?

Do you have the shop manual? It's the best thing next to buying the truck itself. It's helped me more than google at times.

 

I was just reading the pinpoint tests in my Ford service manual (1997 F250HD, F350, and Super-Duty).

What you have described are two internally leaking master cylinders. You're on the right track, as near as I can tell. Just unlucky in replacement part quality.

 

Plugging the outlet helps to eliminate variables to isolate the problem. It has always been my understanding that master cylinder is basically a piston that applies force to the fluid to force it to move through the system to force the pistons and calipers to move. And there should be no pressure loss in the master cylinder. By plugging the ports, or some call it dead heading, if there is no place for the fluid to go, the pedal should move a litte and stop because the fluid can only be physically compressed to a certian point

Perhaps it is just really bad luck on my part that the original cylinder developed blow by and these two maybe have been sitting around a long time and the seals dried out. The wierd thing about the second one is the fluid from the bottle was clear but after bench bleeding it had a red tint, like they had some sort of lubricant that mixed in with the fluid.

 

From everything I've ever learned, you are correct in your understanding of the system. In dead-heading the master cylinder, you have eliminated every possibility except the internal seals/cylinder housing itself.

Are you buying remanufactured master cylinders, or new ones?

Having sold a lot of reman components, I have seen multiple bad ones in a row. It can be caused by something as silly as someone at the reman facility got a promotion, and is learning how to do their job. Or, it can be caused by the remanufacturing facility having a malfunctioning test fixture that they didn't catch until after your parts were put out for distribution.

The other thing that comes to mind is that remanufactured parts are available simply because they have already failed at least once. (Or, they were replaced when they weren't really the problem.) So there may be some issue with the part that the remanufacturer isn't testing for when they determine whether a core is rebuildable, or not.

The only other thing I can think of to ask is how much force are you using on the piston when you bench bleed the new master cylinder. You should not have to use a lot when doing this as there is no resistance. Since brake fluid does not compress (at all - except under huge amounts of pressure), it's not hard to blow out a seal if you are using too much force, or forcing the travel of the piston farther than it is designed to go.

I'm sure that the red fluid is probably some sort of oil to keep the seals from drying out. Hopefully, it's not blood.

Interesting side note I picked up on when reading the service manual earlier - that had not occurred to me before- When a brake pedal feels spongy, it is because the brake hoses, or lines, are deteriorated, and need to be replaced, or there is air in the system. I don't know why I never thought about the brake hose deterioration before.

 

They look like new castings but I am not 100% sure. When bench bleeding it, it does not take lot of force but some because it feels like there is a spring in there to push the cylinder back.

I might try bench bleeding it the new way I was told. Maybe there is a bunch of air trapped in it that didn't come out. I might try to find a used one to put on to see what happens if that doesn't do it, or try another new one.

 

Fluid contamination could cause your problem, like suggested earlier, I would try a different manufacturer/supplier. You don't know if someone put the wrong fluid in, had the same problem, and returned it to the parts store, only to be resold to you.

 

Another good point. Grab your wife's fingernail polish bottle, and put a small mark somewhere on the casting. That way, you'll know if you are getting the same one back.

 

they are definitely different ones because still have all three in my garage, except the second new one that is still on the truck. I just talked to the parts store and said they are brand new and haven't had them for very long.