I have a 96 f250 auto 4x4 diesel and the brake light came on last weekend when I was almost home. I was lightly braking and the brake light came on the pedal felt kind of soft. Backing up and braking I need to push real hard, like there is no vaccum assist but going forward and braking the pedal is inconsistant and feel a little soft. If I put push on the pedal when parked, engine running, the pedal feels like it slowly keeps loosing pressure. I put a new master cylinder on it last night but it actually feels the same or slightly worse. I bench bled it and tried to bleed it some up where the two brake lines come out, left them a little loose and pushed the pedal down to force air out like you would at the caliper/wheel cylinder. I guess I can try and do the whole system but I don't think that is the problem. Could it be a vaccum leak or the vaccum assist part? How would I check. The other strange thing is when I was bench bleeding it, there appeared to be some fluid leaking the back of the master cylinder, around the area where the push rod goes in. Possibly a bad new master cylinder? Any ideas? Thanks.

 

sounds like it is bad.

it should not leak.

if fluid gets into the vacuum assist you will need to replace it also.

 

I agree with that, back of master cyl. should be bone dry.
Also on the diesels there is a vacuum switch which will put the warning light on if the vacuum supply starts to fail. I would guess your vacuum pump is bad. However it could be a leaking booster. I would put a vacuum gauge on and see what the pump is doing. A good pump will do 20in of vacuum. Hope this helps.

 

Last night I got a couple of brass plugs and capped off the outlet ports of the master cylinder. With the truck off, I pushed the pedal and it felt like it is holding pressure good, so I don't think the master is bad. I pulled the vacuum hose off the booster, I don't have a gauge, but sticking my finger over it feels like there is decent vacuum there.

Putting everything back together and pushing on the pedal with the truck running, the pedal at first feels somewhat normal then it feels like it looses pressure, similar to if someone opened up one of the bleeder valves. To stop the truck, either forward or back, takes more pressure, almost like there is no vacuum assist, to stop it. I haven't bled the system but this was pretty much the same symptoms as before I replaced the master cylinder.

What would trip the brake light? Low fluid or low pressure? Is the sensor on the end of the master cylinder to monitor pressure?

 

Only three things that I can think of will put the red light on, parking brake applied, low brake fluid level and insufficient vacuum.
If you have poor power assist but good vacuum then really only the booster is left. Try this, pull the vacuum pipe off the booster again and move truck forward and back and see how the brake pedal feels then. If pedal feels the same with pipe off or on then you have a faulty booster (providing you are confident you have good vacuum). While you do this block the end of the vacuum pipe with a bolt or something. It would be interesting to note if the red light now goes out.
Try that and post back.

 

I tried the truck with the vacuum hose off and the effort stop the truck was about the same as with the hose on. The only real difference is with the vacuum on, the pedal continues to go down, like the bleeder valve is open. With the vacuum off or truck turned off, the pedal does not keep depressing. I did replace the booster last night also but same thing. I don't know, I am lost now. Unless the vacuum pump is going bad and can't keep up but why would the pedal keep going down with the vacuum hooked up, but when the truck is off it doesn't do that. If the vacuum pump was bad wny would the pedal continue to go down, would it stop moving the same way as if the truck was off?

 

With engine running when you press the pedal does it quickly go to the floor? If it goes down slowly under constant pedal pressure you may have a bad RABS valve unit. Post back.

 

with the engine running it goes slowly under constant pressure.

what and where is the RABS valve?

 

The RABS valve unit is the thing that gives you rear anti-lock brakes and is controlled by a computer. It has two electrically operated solenoid valves in it. You will find it on the inside of the left frame rail about level with the transmission. Just look for the brake pipe coming down from the master cylinder that goes right to the RABS unit.
It is quite easy to eliminate it as a cause of the problem. You will see a large hex plug (bung) screwed into the front end of the unit. Unscrew this (under spring pressure all the way) and remove the spring, then replace the hex plug. Pump the pedal a few times to fill the chamber and then see if your problem is solved. If it is you know you have a bad RABS unit, if problem not solved just replace the spring and RABS unit good to go.
The RABS is not much good anyway so if all that solves your problem you can leave it that way or replace the unit. Let us know what happens.

 

So it sounds like brake fluid will come out of it or is it just the cylinder that the spring would push on that fill back up? Would I need to bleed the system? You didn't mention it so I assume not? Thank you very much for the help so far. Also if the spring is left out, that won't trigger the brake light will it?

 

No brake fluid will not come out, there is a piston in there that keeps it sealed. You should not have to bleed it, however there is a bleed screw on the RABS unit if it is necessary. No leaving the spring out just disables the RABS without the computer being able to tell.
It is worth a try, it doesn`t take long to take the spring out or put it back in again. Good luck.

 

I am still having problems, I replaced the rabs and everthing in the rear brakes and still same problems, including bleeding the system. How does the rabs work. I know there are wires off of it but does it just send signals or does it recieve from the computer. Thanks.