Funny you say that when the dealer did front brakes on my 02 Durango they replace the rotors and it was the first brake job, I owned it from day 1 and they knew that.
When I asked why they said when turned and still in spec then tend to warp so to stop come backs they just replace them.
Another dealer many miles away (NC from CT) did the same thing on the 2nd brake job and I never question why.
Truck has 220k on it and I only did rear brakes because of a leaking wheel cly.

On my project F100 I did replace the rear drums along with everything inside them.
Front rotors looked good but replaced the pads & calipers so hope all will be good when on the road.
Dave ----

 

Here it is, a year later, and I'm still fighting these brakes. They were pretty good since my last post, but lately the LR has been locking up easily in the morning on our dirt road. To fix the overly-eager LR brake and poor parking brake action. we just did new shoes and a hardware/spring kit. We also lubed the parking brake cable to that wheel. Adjusted and bled, that wheel no longer tends to lock up. I suspect old return springs were the culprit.

Everything is new except the calipers and metal lines.
New drums, shoes/springs/hardware
New MC, vacuum pump, booster (not rebuilt)
New rubber lines and pads in front
New pressure differential valve (1 wire model)

But we're getting intermittent brake warning light, and poor pedal response. The brakes suffer a delay. You push, the pedal is mushy as if the system were not bled, then about 2 seconds later the brakes start working, front and rear. They feel fine, in fact. But those first two seconds are disconcerting. This caused one of our sons to damage the original XLT Lariat bright trim grill last night (tips welcome on where to find one).

The fact that the brake warning light comes on indicates that the pressure differential valve has detected what it thinks is a leak. That's my understanding of (one of) its function(s). Its little piston moves fore/aft to the end of the truck that has the imagined or real leak and blocks it off. But if that were the case, why do the brakes feel good after a couple of seconds?

Way back in the OP, I mention an old-timer who was a Ford service manager when Bullnoses came out. He said to stand on the brake pedal when stopped, to reset the pressure differential valve. I tried, no luck. Then there is a bleeding procedure he recommended, again back in the OP. He said that bleeding one end of the truck can trip that pressure differential valve. We did, in fact, take that truck to @calvinhg 's shop so he could vacuum bleed the rear. Perhaps we need to bleed the front now, and follow the former service manager's suggestion?

"We talked to an older gentleman who worked at Ford for 30 years. He says 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. "

 

FWIW, I've had mixed results doing a vacuum bleed. I'm not sure exactly why, but process just never seems to get air out 100%. My thoroughly non-scientific hunch is the when the the lines are under a vacuum, this causes any air bubbles to disperse like a foam. Heck if this is what really happens, but I've given up on vacuum bleeding. I've tried all the tricks for vacuum bleeding, such as sealer on the bleeder threads, etc., but no joy. If you have been having consistently poor results, too, I'd consider another method. More about that below.

Did you bench bleed the new master cylinder? I used to think it didn’t matter, but it does, especially on vehicles with any kind of proportioning valve. I have changed master cylinders on older vehicles, didn’t bench bleed, and had no issues. You’re introducing a little bit of air at the start of the circuit, so it just takes a little more bleeding to get all of the air out at the far end, that’s all.

The proportioning valve changes all that. A certain amount of pressure must build up on the upstream side of the valve before it will open and send pressure to the wheels. But if you’ve got any air between the MC and valve, you run into a Catch 22. The MC can only send a finite volume of pressurized liquid. Think of it as a slug of pressurized liquid with some equally pressurized air in the mix.

Now if you were sending an unlimited supply of pressurized liquid to the valve, it would just flow right through and push any air ahead to the bleeders. But unfortunately, that’s not what happens. The MC is only sending a small slug of pressurized fluid/air mix. Before all of this mix can clear the valve, the valve closes as pressure decays at the end of each bleed cycle. This leaves air trapped upstream of the valve no matter how many bleed cycles you try. There’s just not enough volume from the MC. This is why bench bleeding is so important, as it reduces the amount of air that can get trapped upstream of the valve.

What to do?

1) Some proportioning valves have a little override pin you pull out and clip in place. This locks the valve open so any trapped air simply flows through.

2) Use a pressure bleeder like most shops do. Here’s the low budget version I have:



You will also need a cap (not shown) that fits on the MC. This setup lets you send a practically unlimited volume of pressurized fluid through the valve, keeping it open as any air gets forced through.

3) Crack the line fittings open on the upstream side of the valve and bleed from there, just like you would at the wheel cylinder. This gets that air out without having to open the valve. This is very messy, so wrap a rag around the loosened fittings. Then tighten the fittings and complete the bleeding from the wheel cylinders.

 

Never heard that before. The only function of the plunger I have ever read about was to turn the light on. I have busted many brake lines on these trucks, and after the light came on, you could still pump the brake and pump all the fluid out of the busted line.

Here's some info on it. https://www.freeasestudyguides.com/b...ing-valve.html

 

This newfangled vacuum bleeding malarkey may indeed be the culprit. If it were very gentle vacuum, then it would be low risk. But suck hard on there and as you say some air will find its way in. I've only done gravity or pedal bleeding. I do have a pressure bleeder setup somewhere in a box. Time to find it.

I didn't install the MC myself. It was my master tech friend, the PSD guru.

The proportioning valve has no little pin on it, afaik. See it here https://www.carolinaclassictrucks.co...ake-valve.html

I think you're right. There is still air in there. Somewhere.

 

Like Dave, I'm not so sure of this. Here's an excerpt from my '84 factory service manual, describing the three functions of this valve. No mention of any fuse action:

 

I just noticed something interesting in the brake section of the factory service manual. It described two methods of brake bleeding:

1) Pressure bleeding, with a pressurized reservoir connected to the master cylinder.

2) The Michael Jackson method, aka a man and a boy. Okay, they didn't use that exact term. I think they called it manual bleeding, where a helper cycles the brake pedal while you open and close the bleeders.

The curious part? For the first step, both methods said to crack open the fittings at the master cylinder and bleed there. Use a rag to catch the fluid.

Why there? I think this supports my theory about how once the proportioning valve opens under pressure, you can't get all of the air past it before the valve closes again (at least during manual bleeding). By expelling as much air as possible upstream, you'll reduce the possibility of air getting trapped in that section of line.

Shame there was no caution note there, warning of problems if you skipped this step. I guess they figured all mechanics always followed all instructions to the T without question...

Oh anyways, I'd highly recommend doing another full bleed, but this time begin by cracking open the lines at the master cylinder. I'd also suggest doing a pressure bleed, so you get a steady flow through the lines, not interrupted like during manual bleeding.

 

^^^^^


Good info, thanks, Turns out my pressure bleeder setup only fits round caps. I'm going to see that old fellow who worked at a Ford dealer for 30 years. His shop is actually named Good Old Boys

 

You're very welcome. Hope the info helps. I had a Eureka moment last night when I noticed that detail in the bleeding procedure. Fingers crossed that is it.

For the pressure bleeder, I used an old MC cap to make an adapter. I added a hose barb fitting a little bit off-center, so I could still secure the cap with the retainer bail. I trimmed an old gasket in the middle so fluid can flow easily to both chambers.

I've seen other universal adapters that are just a flat plate with a hose barb. As long as the gasket is open in the middle, fluid will flow to both chambers. Securing a flat plate can be a little tricky. I've seen a big C-clamp used, for example. I went with a stock cap so it would be easy to secure.

 

Now it's my turn for a light bulb. Use an old MC cap! Yes! I think we even have one.
But you have to pump fluid into it under pressure. And be able to monitor the fluid level in your device above it. So late, coffee wearing off, must think more tomorrow. Thank you!

 

Not sure what kind of pressure pot you have. Mine is translucent plastic, similar material as a plastic milk jug but much thicker. It's pretty easy to see the fluid level, especially if you hold a flashlight to it.

 

Bumpity bump!

Did you ever try pressure bleeding? If so, did it make a difference?

 

I've thrown in the towel for now, taking it to my master tech buddy this week. We tried Michael Jackson bleeding again, got a little air, but pedal remains as it was. Slow to respond, then good. I'm not sure that we got that differentiating valve in the center, because the brake warning light is intermittent. So we weren't sure the valve was centered when we put in the centering pin. And it has no O-ring, so leaks when you step. My master tech friend has done all this many times over the years.

 

Ah, another little clue. If fluid is leaking out when you step on the pedal, the leak may draw air back in when you release.

This leak is new, right? Maybe it wasn't a factor originally, but has been a part of the equation ever since the leak started. In other words, you have fixed all the original faults, but now this new fault is active and causes the same symptoms. That would drive you crazy troubleshooting.

Is your mechanic friend planning to do a pressure bleed? I'm just wondering, as I've got a hunch that pressure bleeding is about the only thing that works when vacuum bleeding and the Michael Jackson method fail.

 

It also leaks from gravity when the system is quiescent. So any air should go back out....perhaps.

Nobody has an adapter around here. I tried 4 good shops. The only hope is to carve/braze up an old MC cap. I'm discussing it with him. Also the idea of going to a bricknose MC with single plastic transparent reservoir, for which a universal pressure bleeder works.