1991 F-350 Regular Cab Chassis 7.3 IDI 4 x 4

Blew a hole in the rear steel brake line a few weeks back. I was in a pinch so I disconnected the rear line at the master cylinder and had the correct pitch bolt to cap that port off. Drove the truck carefully for two days until I could get all the parts I needed.

My front pads were worn down to replacement thickness. I bought loaded calipers from my local NAPA dealer. I also bought enough steel brake line to replace the entire steel lines on the truck. Time and daylight were running short so, I only replaced all the steel lines for the front brakes and installed the loaded calipers. I didn't replace the front hoses this time because they were replaced two years ago and they didn't have any visible cracks in them. Bled the front brakes and still had the bolt in the rear port on the M/C. Drove the truck the next day because that was the only mode of transportation for me.

The day after I did the above work to the front, it was time to tackle the rear. I replaced the steel lines from the rear brake hose to the pressure metering valve(???) on the drivers side frame rail. From the valve all the way to the M/C. In the M/C, I noticed that I partially crushed the port seat for the rear brake line. I figured that the small price to pay for a replacement M/C was better than having a truck that was down and out.

Another trip to the NAPA store and I had my NEW not rebuilt M/C. Bench bled the M/C and hooked it up to the truck. Bled the whole brake system....R/R, L/R, R/F, and L/F. The pedal was nice and firm.

Then I started the truck up. I stepped on the brake pedal, and it slowly sank to the floor. I put the truck in drive and I would have no problem stopping the truck, except the pedal would sink to the floor. I wouldn't want to try to stop the truck at highway speeds with a pedal like this. I would shut the truck off and step on the brake pedal some more with the same results. Once the vacuum pressure was no longer present the pedal would be hard again.

I decided to replace the vacuum brake booster. Same symptoms as before. Firm pedal with the engine off, but sinks to the floor while the engine is running. No vacuum gauge to check how much, but I had vacuum from the pump when I removed the vacuum hose from the booster. I ended up replacing the vacuum pump with no change in the sinking pedal.

Took another trip to my NAPA for a replacement NEW M/C. Bench bled the M/C and installed on the truck. Bled the brakes R/R, L,R, R/F, and L/F. Still the same problem.

No visible leaks from the drums or the new calipers. The M/C doesn't lose any fluid either.

Any suggestions? I'm leaning towards the metering valve. I saw this link and read it from a previous post....http://www.asashop.org/autoinc/dec99/techtips.htm

 

Time for another master cylinder.

This time fill the master cylinder with fluid and pump it out into a cup.
I bet you get lots of metal shavings out of it.
Quality control is not what it used to be.
The shavings are from the machining process, they just never got cleaned out.
The shaving cut the new seals up in very short order.

Been there, did that.

 

This reservoir is not the cast iron type. It is the plastic type that has to be removed and installed on the replacement M/C. It actually has a little float in the middle of it. I placed the bleed tubes on top of it and I monitor them through the whole bleeding process. Nothing foreign comes out of the tubes, except the air bubbles, while bleeding the M/C.

 

it has a steel sleeve pressed in the center and either way i'm with Dave they manafacture/machine ALOT of parts and never do a proper cleaning/inspection/assembly process. Most likely you'll replace tha valve though if you can't flush the paticles out and get it to re-center.

 

What's "it', the M/C?

"that valve" is the metering valve?

 

yes and...yes , my appoligies

 

Push down on the pedal without the truck running. Does the pedal get hard? If it does, keep some pressure on it for a minute or two. Does the pedal drop or stay put? If it drops then you have a bad master cl. If it stays up and holds then you may have more air. How did you bleed the system? You don't want the pedal pusher pumping the pedal. Push down the pedal and hold it. When you pump the pedal you can foam up the fluid from the air in the system.

Are you getting brake pressure to the front and rear? The combination valve may not be centered.Is the brake light on in the dash?

 

The pedal is nice and firm. No sinking whatsoever. I can practically stand on the pedal with both feet and put my head through the roof of the truck without any sinking of the brake pedal.

The system was bled with one full steady stroke of the brake pedal and then it was released. Fifteen seconds later the pedal was pushed again and held. Next the bleed screw was opened at whatever wheel was in the bleed sequence (R/R, L/R, R/F, and L/F), and the pedal sank to the floor. The brake pedal was not released until the bleed screw was closed. Fifteen seconds later, the sequence would be repeated.

Whichever bleed screw was being worked on, a short length of hose was attached to it and the other end was in a clear jar submerged in brake fluid. Bleeding of that bleed screw wouldn't cease until there were no bubbles appearing and it was a clear steady flow of fluid. The M/C was never allowed to get below halfway filled.

On a side note, after the brakes have been bled, the M/C reservoir wasn't filled up to the full line, I had the pedal pusher push and hold the pedal for me. I did notice when the pedal was released, some brake fluid would briefly boil up in the rear of the reservoir. No air bubbles, just fluid.

 

Have your pedal pusher pump the pedal 5 to 10 times in quick succession hold down hard then open the bleeder do this on all 4 corners.. keep the M/C filled.. look over all your lines you made up for leaks while the pedal is held down.. you still have air in a line someplace.. work it out.. its not the booster its hydralic

 

Like I said in my original post....there are no visible leaks whatsoever. The replacement lines are one piece except for the two 72" lines that go from the metering valve to the rear brake hose. No wet spots at any of the flair nuts, ie. master cylinder, right or left front brake hoses, rear brake hose, junction between the two 72" rear lines, or the rear brake hose to each wheel cylinder.

The truck has been driven a few times, about a mile in total. Nothing coming out from the rear drums. The frame is dry. The fluid level does not drop in the M/C reservoir.

I have gone through four full quarts of brake fluid and the fifth is almost empty! I have bled and bled the brakes over and over to no avail.

 

***UPDATE***

I plugged both ports to the M/C. The brake pedal was solid and no sinking with or without the engine running.

I attached the rear steel line to the M/C and bled the brakes in the following order; R/R, L/R, and the metering valve. Nice solid brake pedal and no sinking with or without the engine running.

I attached the front steel line to the M/C and disconnected the crossover steel line from the L/F hose/distribution block and plugged it to disable the R/F caliper. With the L/F caliper bled, the pedal was solid without any noticeable sinking. I cranked up the motor and tested the pedal again. It was slowly sinking.

I couldn't test the R/F caliper on it's own because the L/F hose also doubles as a distribution block for the two front brakes.

Once again, the front hoses were replaced just over two years ago. They are nice and solid, not spongy. No visible cracks either.

I read something on this board the other day when I was doing a search about someone with a similar problem with a dual piston setup. Does that make any sense? No brake fluid loss, but the L/F dual piston or both calipers creating the problem?

It is a NAPA rebuilt loaded caliper(s). I did at one point, while running with front brakes only, have to practically throw the anchor out to stop the truck, but that was once. I was rolling at 45 - 50 MPH and had to stop.

What does everyone else think? Is it plausible?

 

Booster?. I had a truck once that it had about one or two brake applications than the pedal would go down wouldn't stop you. It ened up being the booster but I hadn't ever seen a booster fail the way this one did. It held vacuum, seemed fine. Buy all the diagnostic procedures for a booster that you could ever read said it was good.

I don't work on juice brakes much because I am in the air brake vehicles or heavy equipment world but it was strange.

 

The original and replacement Vacuum Brake Booster acted identically on this truck. The replacement one was a NAPA rebuild.

 

You also replaced the booster? I didn't
see where you did, I must of missed it.

Does this have rear anti-lock?

The lines are in the correct spot as far as front to rear circuit on the "metering" valve? I am not trying to be smart be we all mess up sometimes.

I am not the best at juice brakes as far as "vehicle specific" problems. Some vehicles have valves that act differently then other vehicles when it comes to failure symptoms.


Evidently something was missed.

I can tell you it's not the wiper motor.

 

You missed the booster part in the first posting.

No lines are in the wrong spot.

The front port on the M/C feeds the front brakes. From there the steel line goes to the L/F brake hose/distribution block. One side of the block contains the L/F hose and the other has the steel cross over line that goes to the R/F brake hose.

The rear port on the M/C feeds the back brakes. From there the steel line goes to the metering valve located on the drivers side frame rail. Out from there the steel line goes all the way back to the cross member where the rear brake hose is attached. At the other end of the hose is another distribution block that allows two steel lines to go to each rear wheel cylinder.

The wiper motor was replaced last December!