Truck
1987 F350 6.9 diesel
Dual wheels 4wd

Problem:

When you are driving and step on the brake it goes to the floor. Let it go and pump it once or twice and it feels like it should and doesnt drop. Does it every time.

Rear brakes adjusted properly. No leaks. Can get a hard pedal that doesnt drop when the truck is off and there is no fluid loss.

Very good vacuum at the booster and the vacuum pump was replaced last year.

Ive had a problem like this before on other trucks and its always been the master cylinder. Replaced it with a new one, bench bled correctly and same problem.

Bled the brakes all around and got no air at all.

Any suggestions?

 

Did you bleed the RABS module on the left side frame rail?

 

yes.............

 

Certainly would seem like air in the system. The proper bleeding sequence is right rear, left rear, RABS module, right front, left front.
Maybe try bleeding them again and close each bleed screw before the pedal is released every time.
It is possible that the RABS module is bad but sounds more like air.

 

Im used to a spongy pedal if there is air in the system or one that wont pump up or sinks.

Im going to take your advice and bleed it out in that sequence and see if that helps.

Thanks for the response!!

 

Worth trying. Post back with the results.

 

no good.....

 

I've had that on some masters.
There's a check valve in there that hold the cyl fluid volume.

If that gets stuck, you hit it a couple times, the check valve loosens up, grabs and holds the correct amount of fluid and works normally.

I've torn them apart to rebuild and spray or actually run brakeclean thru them , blow them out real good and they may work ok.
..but you put in a new master? bad out of the box maybe?(rare)


There is also something intertesting about your brake boost vacuum.

You say engine off you have stiff pedal from the first pump?

engine on and after a couple pumps, it comes up and is ok, right?

so wondering how a vacuum could suck, say, the master piston just slightly enough to set the seals just a hair beyond it's fill range.

I've also had that happen when a pedal won't come ALL the way back up..like a simple brake pedal return spring...and I'm talking about the last godblessed 1/4 inch.

The pedal and master piston need to be all the way up to be able to grab enough fluid for the next push.

then again, I was driving around for weeks with my brake pads clips ready to fall out so I question my own advice. :-)

good luck

 

Im wondering if it is a bad new master. I get good vacuum. The vacuum pump and the booster were replaced last year. Ive run dam near a gallon of fluid through it bleeding and did not get one single air bubble out of any fitting.

Basically youre driving down the road and hit the brake. It goes right to the floor. You let up and hit it again and it feels like brakes should.

First pump, nothing
Release and hit the brake again, it feels fine.

No fluid leaks anywhere and a rock hard pedal with the truck off.

 

I've bled to death too.
Even ran aquarium lines from the bleeders back into the master reservoir and discovered the 'fill' issue.


Next time, try holding UP on the pedal with your foot beFORE you hit it for the 1st time when it's running.

The master piston seals need to be ALL the way back in order to grab a full cylinder of fluid from the fill valve.

Why it's different between vacuum and no vacuum is the mystery to me.

Yeah, try holding UP on the brake pedal before you hit it for that first time.

also maybe the master is sitting too deep in the booster mount?

was there a spacer or gasket or 'o' ring or anything that would set the master at a different depth to alter the throw of the pedal.

New master acts exactly as the old one right?

 

I tried what you said. No different.

And yes, new master just the same as the old one.

Brakes were fine on the truck and one of my drivers came back and thought it blew a brake line. Thats how it felt.

Ive felt the same symptom on other trucks many times and barring any leaks, its always been the master. So Im really questioning if I have a bad replacement.

Im not one to be a "parts changer". I thought I had this diagnosed right

 

The only other oddity I've seen is for a master to leak 'internally'

or more accurately, I loosened the bolts to remove the master and the brake booster was full of brake fluid...or ripped seals that just leak past themselves and Not to the outside.

Wish I had the answer but like you said, to find someone who has actually been thru this exact scenario is what you need.

 

Could be a RABS problem too, like a dump valve not closing properly. Read here on how to lock the piston to test for a faulty valve:
https://www.ford-trucks.com/forums/9...ml#post9433272

 

Im going to try bypassing the rabs unit

 

Bypassed the RABS and still the same problem.

Im really wondering if I have a defective master