Boys and Girls,

After witnessing much wailing and gnashing of teeth when it comes to bleeding the brakes on our old trucks, I wanted to pass along what I've learned. The typical thread starts something like this:

"I just replaced the (insert name of brake system part) and tried to bleed the system, but now the pedal is kinda mushy. It wasn’t like that before. I've tried bleeding 16 times. My neighbor says we should try a 17th time. What do you think?"

First, let me share what doesn’t work very well:

1) The Michael Jackson method: AKA a man and a boy. This is the old-fashioned method with a helper holding and releasing the brake pedal while you cycle the bleeder valve at each wheel. On other vehicles this usually works well, but for reasons to be explained later, not so much on our trucks.


2) Vacuum bleeding: A miserable method using a handheld vacuum pump, such as a MityVac. Personally, I’ve given up on this method for any vehicle. Drawing a vacuum from each wheel cylinder sounds great on paper, but there’s something about the process that creates several bazillion tiny little bubbles dispersed throughout the system. Good luck getting them all out. It just seems to be an exercise in futility.


The main issue seems to be how fluid behaves passing through the pressure differential valve. This little creature is near the master cylinder, located just downstream. On my ’84, the valve sits on a bracket above the driver’s side frame rail, directly below the steering shaft. Follow the lines from the master cylinder to find it. This misunderstood valve is often called by many names, such as a proportioning valve, a metering valve, or a #%$@ valve. The pressure differential valve has three distinct mechanisms in a common housing. Here’s an excerpt from the factory service manual describing the three functions:

1) Metering valve – Delays full effectiveness of the front brakes

2) Warning system – Turns on the warning light

3) Proportioning valve – During heavy brake application, it reduces pressure to the rear brakes to avoid lockup







For our purposes of bleeding the brakes, we’re only concerned with the metering valve portion. Straight from the horse’s mouth, as seen above:

“The metering valve delays the effectiveness of the front disc brakes until the rear brake shoes are against the drums.”



Sounds all fine and dandy, but what that does mean? Well, disc and drum brakes have different reaction times when hydraulic pressure is applied. With discs, there is essentially no gap against the pads. Apply hydraulic pressure and they start clamping almost immediately. Drum brakes are a different story. With the brakes off, the shoes are retracted a fraction of an inch. When hydraulic pressure is applied, the shoes must first move against the drums, away from the retracted position. This extra motion creates a slight time delay. If full hydraulic pressure was applied at the same instant to all wheels, the front brakes would grab well before the rear shoes were even in place. Not good, especially with a heavy load. The metering valve overcomes this by sensing the pressure in the rear brake lines. When brake fluid is initially sent to the rear brake cylinders (with or without a metering valve), pressure stays low until the shoes have moved into position and meet some resistance. Now the pressure in the rear lines will start to rise. The metering valve senses this pressure rise, interprets this as the shoes being in position, and allows full pressure to the front brakes, too. Until then, hydraulic flow to the front brakes is reduced. The flow isn’t shut off completely, but is restricted.

Hopefully, the little cartoon light bulb should appear above your head with that last bit. The metering valve (inside the pressure differential valve) limits hydraulic flow to the front brakes until sensing a certain amount of pressure to the rear brakes. Below that threshold, good luck getting all the air out of the front lines. This is where you run into trouble bleeding the brakes the old-fashioned way.

Say, for example, you need to replace the master cylinder. Bench bleeding the new master cylinder is VERY important. I used to think it didn’t matter, but it does, especially on vehicles with any kind of metering 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 the air out at the far end, that’s all.

The metering valve changes all that. A certain amount of pressure must build up in the rear lines before the valve will open and send full pressure to the front wheels. But if you’ve got any air between the master cylinder 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.

If 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 master cylinder 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. But even that won’t get all of the air out.

What’s a poor guy 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. IIRC, this is only on lighter duty models. My F250, or example, does NOT have this feature.

2 - Crack the line fittings open on the upstream side of the pressure differential 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.

3 - Use a pressure bleeder like most shops do. Here’s the low budget version I have. This thing is slicker than Bill Clinton under oath:


https://www.motiveproducts.com/colle...c-bleeder-kits


That page shows the same basic pressure pot with adapters for different vehicle applications. I made my own adapter, but it looks like kit 0105 would work on our old trucks.

Here’s my homemade adapter, made from an old cap for the master cylinder. The center of the gasket has been trimmed away to let fluid flow easily to both sides of the reservoir. The old cap had a vent, which I sealed with epoxy:





Here’s the pressure pot itself. It comes with a threaded connection for the different adapters. I swapped the threaded fitting for a quick release fitting, the type used on air hoses, but this modification is not required. You pump the T handle to build up pressure, much like a garden sprayer.





This view shows the inside of the pressure pot. The exterior flex line is connected to a rigid line that reaches down inside, almost to the very bottom. When the pot is pressurized, fluid flows up into the line:





Here’s my own little gallon-sized invention, something to help reduce the misery of brake bleeding. I got tired of trying to catch the expelled fluid into some container that would just flop around and occasionally tip over. This holder is a piece of ¾” particleboard, about 15 inches square. I chose particleboard because it’s heavy. The empty milk jug sits in a box made of 1 x 4 lumber on edge, secured to the particleboard with screws from underneath. Make the dimensions fairly snug to grip the milk jug, otherwise it will tend to still flop around. Even if you don’t want to be bothered with all this pressure bleeding rigmarole, fabricate something like this for catching the spent fluid. It makes the process MUCH nicer:





For the cap of the milk jug, I got a little carried away and added a brass bulkhead fitting to grip the flex line connected to the bleeder port. You could simply drill a hole and that would probably be adequate, but make it a snug fit so the line doesn’t work loose. Because the flex line fit tightly, I installed the fitting slightly off center and drilled a small hole for a vent. It’s important to have a way to let air escape as you fill the jug while bleeding:





Here’s an overview, showing the pressure pot, homemade reservoir adapter, and milk jug catch container:





The excitement is building, isn’t it? Well, before you do anything else, chock the front tires, jack the rear axle slightly off the ground, and install jack stands. Put the jack stands under the frame, not the axle. This allows good access to the adjusters from underneath the truck, without the jack stands in the way. Remember, the tires need to be slightly off the ground. Put the transmission in neutral and release the parking brake. At the bottom of brake backing plates, you will find an oblong rubber plug. Remove this plug and manually adjust the rear brake shoes. In theory, the rear brakes are self-adjusting, but this doesn’t always work properly. It’s important to have the rear shoes retract just the right amount, but no more. I’ve had good results tightening the adjusters until the wheel won’t quite spin by hand, and then back off 5 or 6 teeth on the adjuster wheel. The official specs call for 10-12 teeth, but I prefer a little less slack and have had no issues. Remember, too much slack means the pistons in the wheel cylinders must travel further when hydraulic pressure is first applied. More travel equals a longer delay before the front brakes get full hydraulic pressure, explained earlier.

Proper adjustment of the rear brakes is VERY important before starting the bleeding, so please don’t skip this step. Here’s a good video explaining the process, tyrap optional. The subject vehicle is a Jeep, but the general principle is the same. The video starts with the drums removed for a better view, but around 3:00 he shows how to adjust the shoes with no need for any disassembly. Remove the rubber plug for access, and that’s it. The process is actually very quick:




With the rear brakes properly adjusted, it’s time to move on to the actual bleeding process. Remove the reservoir cap and use an old turkey baster to draw out the existing brake fluid into a container for disposal. There’s no point in pushing that old stuff through the system, so top off the reservoir with fresh fluid from a sealed container. Install the pressure adapter in place of the cap on the reservoir. Fill the pressure pot with a quart or so of fresh brake fluid. Here’s a view of the pressure pot next to the master cylinder, complete with my homemade adapter. Note the puppy training pad underneath the master cylinder, to catch any fluid spillage when removing the adapter:





Important: Note the two orange C-clamps securing the adapter/cover to the top of the master cylinder. My original plan was to simply secure the adapter with the stock wire retainer. Bad idea. When pressurized, the adapter lifted up ever so slightly. The wire retainer didn’t offer much resistance. Did I mention the reservoir was now full of pressurized brake fluid? Not my greatest moment. The clamps worked so much better. Didn’t leak a drop.

The actual bleeding process is kinda anti-climatic. Pump up the pressure pot to about 15 PSI. You will see fluid move through the clear line to the master cylinder. Don’t worry about any air bubbles here, as they will harmlessly collect above the fluid in the reservoir. Start with the rear brakes, and connect your catch jug to the bleeder screw. Crack open the bleeder slightly and let the fluid run until clean and free of bubbles. Make sure the pressure pot doesn’t run dry, but it’s easy to see the fluid level through the translucent plastic. After bleeding the first rear brake, move across to the other side. This brake will bleed quickly, as you have already cleared the long run between the master cylinder and rear axle.

Next step is to bleed the front brakes. Once again, make sure the pressure pot still has an adequate fluid level, but don’t fill it too high, as explained below. Pump up the pressure pot occasionally, as needed. Here's an action-packed shot of the collection jug in use. Note how the jug is sitting upright, not on its side, a marked improvement over all previous brake work:





After all 4 brakes have been bled, it’s time to disconnect the pressure pot and adapter, which must be done in proper order. Ideally, you’ve got an inch or so of fluid left in the pressure pot. Tip the pot to the left (when facing the gauge on the front) to get the internal intake pipe above the fluid level. Briefly crack open a front brake bleed again. With the intake pipe clear of the fluid level, air will now be pushed through the line. The idea is to let the reservoir level drop slightly, but without running it dry. On a vehicle with a translucent reservoir, this part would be easy. On a cast iron reservoir? You’ve got to guess how much fluid to bleed off. Don’t worry, it’s quite forgiving. Repeat the process with a rear brake, so that both sections of the reservoir are not overflowing. Now you can relieve the pressure, by slowly unscrewing the cap of the pressure pot.

With all pressure released, it’s time to remove the adapter from the top of the master cylinder. This is where I recommend keeping some rags or absorbent pads underneath, as you may get a little spillage. Hopefully you guessed correctly when bleeding down the fluid level in the reservoir, and spillage will be very minor. Top off the reservoir with fresh fluid, reinstall the cap, and you are good to go. Okay, you’ll still have to set the rear axle back on the ground, but I think you’d have eventually figured that out on your own.

Important: With any brake work, always test carefully for proper pedal feel and brake response before driving away.

Here’s another low-buck version of a pressure pot, made by forum member FuzzFace2. See post #6:

https://www.ford-trucks.com/forums/1...l#post19203249

 

You know what the fancy pressure pot thing looks like to me in the picture below? A garden sprayer you can get at the hardware store.

 

Agreed. I had considered using a garden sprayer and building my own brake bleeder. Two reasons against:

1) Not sure how everything would hold up to brake fluid. Might be okay, but who knows. The seals on the pump would probably be what gives you trouble. The plastic tank body would likely be just fine.

2) I liked having the built-in pressure gauge.

 

Nice write up. Your writing style is always entertaining as well.

 

So the official pot floods the master with brake fluid? How do you bleed the master itself? I thought it just put air on top of the fluid. But you would have to keep taking it apart to check the level of the fluid. I never used one of these obviously. Do you put the plate on and clamp it tight, then pour fluid in the quick fitting to fill the master to the top and push the air out? And then bleed the pump and the line till it's full of fluid and then push the quick disconnect onto the fitting? Thus trying to get most of the air out of your pressure rig assembly?

 

Easy solution to the fluid mess problem. Add a straw to the fabricated cap that extends below the reservoir fluid level in the rearmost (front brakes) reservoir compartment.

1) Drain then fill reservoir with fresh fluid.
2) Connect fabricated cap with straw extending to below the fluid level.
3) Pump up power bleeder. As pressure rises, fluid is introduced into the reservoir. the fluid level rises, compressing the air in the reservoir. @ 15 psi, one atmosphere, the air will be compressed to half its original volume.
4) Bleed brakes
5) Release pressure by loosening the cap on the power bleeder pump. As the pressure drops, fluid is expelled from the reservoir through the submerged straw and back into the pump. The compressed air in the master cylinder returns to its original volume and the fluid is below the flood rim just as it was when you started.

 

If replacing the master cylinder, you'd still have to bench bleed it before installation. Maybe you could get away without it, but I don't really know. Seems you'd need to move the piston back and forth a few times to get out all of the trapped air.


Nope, the pot sends fluid under pressure into the top of the reservoir. There's a little bit of air in the flex line at first, but this small amount just joins with any air already in the reservoir and forms a small bubble above the fluid. It's not a problem at all. As you extract fluid from the bottom (via the brake lines and bleeder screws), the reservoir level stays fairly constant because new fluid automatically enters from the pressure pot. The new fluid (from above) just passes through the air bubble. The bubble never gets pushed down into the brake lines. The bubble just stays at the top. It would be interesting to watch this on a translucent reservoir, but I doubt the fluid level changes much because of the constant replenishment from the pressure pot.

The only time you deliberately pump air into the reservoir is just before removing the adapter cap. You tip the pressure pot so the rigid pick-up tube is above the fluid supply. Now you get pressurized air instead of fluid. When you open a bleeder screw, the reservoir level will now drop. You just need to drop the level a little bit so the reservoir won't overflow when you remove the adapter cap. Don't go too low or you'll pump air into the brake lines and have to start all over.

The process may sound complicated, but it goes very smoothly. It cured my truck's mushy pedal with too much travel.

 

Here is my home made psi bleeder.
Screw in tire valve stem with the core removed. The hose from the holding tank goes onto the stem.

IIRC 3" PVC pipe & caps, drilled and taped for the pipe fittings.

Mine uses shop air turned way dome to 10 psi.
As for the mess when I went to remove the cap I never thought of turning the unit upside down and bleed the brakes a little till I did not see fluid in the supply hose to the cap.
Now I know for next time.
I did buy a cheap garden sprayer and I was modding it to use and turned it into junk so it is sitting on the work table till ????????
Dave ----

 

Much appreciated Karl! Super pumped (puns) to try this out ...right after I replace my front rotors and press in new wheel studs.

 

Those can be turned.

 

That photo was last year

 

heck just throw pads on it and roll on
Dave ----

 

Been through a set of pads already in the last year. Only runs down the road for trash cans, Home Depot, and/or dump runs. So about 400 miles I would guess. My wheel and tire are caked in brake pad dust lol.

 

Hi Karl,

I´m doing my first pressure bleed. I´ve replaced wheel cylinders in back, calipers in front, new lines & rubber hoses back to pressure differential valve. New master cylinder (bench bled) , original lines from master cylinder to pressure differential valve were not rusty like all other lines so I didn´t replace them. Rear brakes bled nicely. I only get a few drops of fluid on passenger front bleeder screw and none on drivers side. What are your thoughts?

Could the pressure differential valve keep fluid from going to front calipers?

Thank you!

 

That's the most likely cause. A few things you can try:

Check if the brake pressure warning circuit has been tripped. Chock the tires and release the parking brake. Setting the parking brake turns on the same warning light, so you want the lever released. Turn the key to Run (no need to start) and see if the warning light is illuminated. If so, that means the valve has detected a major pressure differential between both hydraulic circuits. I'm not sure if the valve can actually shut off the fluid flow to the side it thinks is leaking, but it might. IIRC, once the warning circuit is tripped, you turn it off by fixing the leak so both hydraulic circuits can build up normal pressure when you press the brake pedal. In your case, maybe you just need to press the brake pedal to reset the valve.

If no help, reapply pressure from the pot and crack open the fittings one-by-one to see if pressure is being blocked somewhere. Could be your new MC is bad or something like that. Use light pressure (10 PSI?) and wrap a rag around each fitting as you go.