Brakes work...then they don't...then they do
 

Hi guys,

Gotta puzzler problem with our Bullnose. The problem is weak braking, the brake warning light coming on, and the rear brakes obviously working harder than the fronts. The rears lock up too easy. The vacuum pump, booster, and master cylinder are all new or reman, 2 months ago. Everything was fine then until last week, when the brake warning light started coming on. Then the brakes started acting up.

My suspicion is the pressure differential valve, on the driver frame rail just below the master cylinder. It's part of the dual circuit master cylinder system. It does have a wire on it, and it does trip the brake warning light. And there is a procedure for re-centering it, according to Chilton's, and that is to stand on the brakes while standing still. We tried that last week but it didn't work. See the Chilton page attached below. Has anybody every had one fail? Were the symptoms like mine?

We talked to an older gentleman who worked at Ford for 30 years. There is a bleeding procedure that can usually restore normal operation, if the brake warning light is working. The pressure differential valve has a secondary duty of shutting off pressure to a circuit that has a leak. So if it's only braking on the rear, the valve has decided that the front has a leak. This could caused by a genuine leak, or by air in the system. Once the pressure differential valve has tripped to shut off one end of the truck, it needs to be re-centered. To re-center it, he said to use the brake warning light. Step lightly on the pedal with the brake warning light on. Crack a bleeder on the end of the truck that IS braking. When the warning light goes out, STOP bleeding. He says that will re-center the proportioning valve to normal op. We were about to give it a shot, but the brake warning light was not lighting up, and the brakes are working well today. Go figure.

Anybody ever had an intermittent brake problem like this one?
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Why would the differential valve get out of center? The only reason would be a leak in a line somewhere, and it would lose pressure on one side.

Yep, that makes sense. That's why we had a look for leaks, but found none. And the fluid level in the master cylinder is right up near the top. If there were a leak, the fluid level in the MC would go down.
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I wonder if 1 side of the master went bad would cause that?
I have never seen it happen but never know.
Dave - - - -

If the brake fluid level is not going down and the brake light is on and you you are only getting one circuit worth of brakes the master cylinder has gone bad. One circuit in the master has failed, time to replace it.

Yes, that is the logical solution. I just was hoping somebody else had experienced similar intermittent problems. Yesterday the brakes were fine, this morning it's back to rear brakes only, with no warning light.

Today we're installing a new master cylinder, to replace the reman in there.

I'll report on how it works.

We got that new MC in there, no improvement! So we replaced all the rubber brake lines, no improvement!

We're tearing out our hair now. I'm gonna look like Brian Stelter. That pressure differential valve appears to be a Ford only item, and the dealer says he can't get it.

. You may have some debris in the line blocking the inlet to the proportioning valve. The problem is obviously between the MC and the proportioning valve. As it is triggering a no pressure in one circuit.
It is the front brakes that are proportioned and this is done after the the fluid sees the plunger for the switch.

Did you get good full pluses of brake fluid out of the calipers when you bled the system it after installing the master cylinder ? Did you bench bleed the master cylinder.?

I took the truck to a master tech and left it with him because I bought an 04 F-350 from him. He listened to the symptoms and all the repairs/inspections we'd done. Right away, he said his suspicion is the pressure differential valve. So he's going to replace that and I'll let you guys know how it turns out.

Aaaaaaaand, it was the pressure differential valve. Failing to find one to buy, we decided to take it apart and have a look. There's a little hourglass shaped critter in there that had left its normal position in a bore. A good cleaning and polish with ultra-fine emory cloth, and it seems to be fixed.

A few more morning test drives, successful, and I'll declare victory. You could probably follow this path too if your Bullnose brakes act up.

How did you take yours apart? I picked one up at the jy the other day since I wanted a backup and would like to clean it up before I seal it in a bag.
I took the sensor out and the nut off but that just had a spring and rod behind it. The shuttle thing would be going perpendicular to these two removable items anyways. One end is solid and the other is a brake line female.

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We did not take pictures, alas, in a rush. I'll huddle with Gary (the master tech) and show him your pics, and ask for more detail.

It is with bitter dissatisfaction, like a bite of lemon pie where your wife forgot the sugar, that I must truthfully report this sad fact.

THE PROBLEM CAME BACK. Without driving !

We parked that pickup for a week because I need to put the clamps on the injection lines. Yesterday I decided to move it over to the trickle charge area, and the brake warning light was on as soon as the IDI fired up. Oh crap. Then the back brakes locked when I touched the pedal. Same ole problem. Our quick rebuild job (clean, polish, reassemble) on the pressure differential valve did not do the trick.

It's very curious that the brakes worked fine last week. We drove to town and got diesel. They were working fine when we parked the truck. Somehow, just sitting there, that #@%&#! valve got screwed up all by itself.

Now I need to scour the internet and find somebody who rebuilds them professionally. Any tips welcome!

Ok what is causing the valve to move and set the light off?
Normally there is a leak in the system to cause the valve to move and set the light off.
Now if you are not seeing any leaks what else can cause the valve to move to set off the light?

I am thinking a bad master can cause that, is that the "rebuilt" part you said in the first post?
I also take it the master & booster where not bought as a package, bolted together and 1 price paid for both?

If not as a package I think I would try and shorten the rod between booster and master.
I say this because I think the master is not returning back all the way and only 1 port is staying covered, not letting fluid out of that part of the system.
You now push the pedal and 1 side is full the other side is not and the valve moves and sets off the light.

The only other thing would be 1 side of the master is not pushing fluid all the time and is moving the valve and setting off the light.
Dave ----