Hi all,

I have a really soft brake pedal and so far I've flushed out the fluid, replaced the brake master cylinder (bench bled with a kit and rebled the brakes), adjusted the rear brake shoes and at the moment I can alteast pump the pedal a few times to build up some pressure but the truck does not give me much confidence with its stopping power.

I noticed in the Haynes manual it calls for pulling the pin out on the brake pressure differential valve before bleeding the front calipers but I can't either pull it out or push it in any with moderate force (it will jiggle a little bit so it doesn't seem "stuck"). I wonder if there's something going on with this part that is causing much of the soft pedal?

It feels like a bad brake master cylinder where you push right through the front circuit before hitting the rears. According to the picture of a similar one I found online it looks like you can take this puppy apart. I was thinking of trying that to see if some gunk was stuck in there causing my problem but wanted to check with y'all first. Thoughts?...

(I don't loose brake fluid, it doesn't leak all over anything that I can see, I was able to use a handheld vacuum pump to suck fresh fluid through each bleeder screw, etc.)

The pin is "C":

 

I have never had to mess with the pin, or ever had a proportioning valve give me any problems. And I have had a lot of stuff go wrong with the brakes on different trucks.

One thing to check is to make sure your bleeders on your calipers and rear wheel cylinders are at the top, higher than the brake lines where they connect. If for some reason the bleeder is low, you will never get all the air out.

And for the rear cylinders, you really have to use a lot of fluid to get all the air out, more than the master cylinder will hold, so you have to stop and refill.

Pumping the pedal to make it rise still sounds like there is too much travel in one of the brake systems, and it's usually the rear shoes. It's simple really, the shoes are too far away, so one stroke does not move the wheel cylinders enough to make contact. So you pump and pump till the shoes make contact. After that, the springs pull the wheel cylinders back in inward, and you lose it all again. Sometimes a good hint that this is the problem, is too pull the parking brake. This will take up some of the travel, and the pedal will be a little bit higher if you leave the parking brake on while you push the brake pedal.

 

Thanks for the quick reply and tips. I'll try the parking brake idea next.

So what may be happening is there is an issue with the rear drums or lines that causes there to never be enough pressure in that line to let the porportioning valve engage the fronts. That must be why the truck can barely stop during normal driving, I'm using all rear brakes.

 

`Does your vehicle have a power booster ?

 

Yes, there is a brake booster.

 

Then check your rod distance from boster to master cylinder. Have someone very gently step on brake while you watch fluid in master cylinder for bubble. Bubble should appear about 3/8 to 1/2 inch of brake peddle being depresed. Adjust booster rod accordenly. Let me know your outcome.

Sctrt

 

I have bled the front calipers with easy, predictable success. I have replaced the master cylinder. I have rebuilt and adjusted the rear brakes. I did not replace the rear brake cylinders, but they are not leaking. Every time, HUNDREDS of times while bleeding the brakes, there is a 1 to 2 inch chamber of air immediately followed by a column of fluid.Tighten, loosen,repeat… the same thing for literally hundreds of times. I have not let the master cylinder go dry.
I am wondering if the prop valve can be sucking air into the rear lines.
there is moisture at the top (where the plug enters) and some at the bottom (with the little rubber nub thing).
my brake pedal goes all the way to the floor with negligible breaking power. It is always slow to return the pedal to the upright position.
my gut says the prop valve needs to go. Is it safe to delete it?

 

It is there to balance the breaking front to rear and also turn on the "brake" light when the PSSI between front & rear is different like a leak from a bad line.
The front dose most of the breaking as the weight shifts to the front when breaking so a little less breaking to the rear helps to keep the rear tires from locking up even more so when empty.
Later years they added an adjustable valve out back that was hooked to the frame & rear axle. When empty it would cut the breaking even more and when loaded and the frame came closer to the axle you get more rear breaking.
Over the years the valve would go bad and not get any rear braking.

If you dont care for the light I dont see why you could not pull that plastic switch and install a plug just make sure it dose not go in too deep and it will keep the valve from moving when needed.
You are better off finding a good one and swap it out.
I dont know how you are bleeding the rear brakes but I have found the bleeders can suck air in and make it look like air is in the system.
With a helper pump pedal 5 times slowly or the master will not fill, and hold they tell you they are holding, open the bleeder and they keep their foot to the floor till you close the bleeder and you tell the to pump 5 times again.
I do this 3 or 4 times at each wheel but before I move on to the next wheel I check the level and top off as needed.
Dave ----

 

I admire your perseverance. Most people admit defeat after about 20 attempts at brake bleeding. There's a quirk with brake bleeding due to how this valve works. Details here, including more info about the three very important functions of the valve:

https://www.ford-trucks.com/forums/1...-bleeding.html

Should you just delete and bypass the valve? How are you under pressure, testifying under oath in court after slamming into a bus full of nuns and orphans? The valve is there for a very good reason. On the slim chance the valve is bad, replacements are available here:

https://www.carolinaclassictrucks.co...ion-valve.html

 

In order for air to enter, fluid must be leaking out. A brake system must be fluid tight. If you are losing fluid you need to chase down the leak. One place to eliminate is the brake booster can. Sometimes the master can leak putting fluid into the vacuum can. The hidden leak.

For the rear bleed adjust the brakes all the way into the drum. After your bleed adjust the brakes for a light drag.

The rear proportional valve limits fluid from the rear brakes unloaded. When loaded more flow to the rear. It might help to load up the rear for a bleed? I haven't done that before but when having trouble bleeding it may help. Is the rear proportional valve hooked up?

 

On the later trucks they went to RABS and the factory did away with the proportioning valve. The RABS system frequently fails and is difficult to get parts for. So I bypassed the valve on my 89. It's like driving any older truck with the single master cylinders. Normal operation you do not notice any difference. If you have to get down hard on the brakes in a panic stop, you can feel and hear the rear tires sliding.