No worries... I think this discussion has helped me figure out the problem; at least thats what I am predicting.

I bet this new prop valve wont work although I will try it. My hypothesis is the prop valve is misreading the pressure signals coming from the rear wheel cylinders which I did not change. Its essentially telling the system its out of adjustment.

In a manner of speaking I think I threw the baby out with the bath water... I need to replumb the h-block and hook the mc to it as bama indicated. I still have my block and lines and fittings to undo what I did but it will be a bit of a pain in the ****. Oh well live and learn. I will let you know.

 

Btw... Anyone have an h-block diagram so I dont jack the reinstall up?

 

Crazy96863, Can I get you to try one thing.
From what you described, you did not adjust your rear brakes correctly. If you adjusted your rear brakes until they just began to drag on the drums, would be way too loose to have a good pedal.
To center the shoes and adjust properly, run your adjuster until it is too tight to turn (shoe very tight against the drum), then start backing off the adjuster until you can turn the tire with your hand with a slight to medium drag.
This will make a big difference in the adjustment.
Note: to back off the adjuster you will have to insert a small screwdriver to release the ratchet and allow the adjuster to turn backwards.

 

Ya I can try that... If it works it saves me some extra effort. If not its only a few minutes of time. I will do this before changing anything elseand let you know. Thanks!

 

'76 F-100, after shoes, pads wheel cylinders etc etc my brakes were good but at a light the pedal would continue to go down. Replaced MC and that stopped that. I bench bled it for what seemed like umpteen times and would still see small air bubble in the port at the bottom of the MC, finally said hell with it and installed it and bled system, so far so good. The advice about the drums is correct, adjust until the wheel locks and then back off until they just drag a bit, if not, you wont have good pedal.

 

Adjusted the drums as suggested; they were, if anything, too tight. Still I tightened then backed off as advised. Nothing.

I replaced the prop valve. Nothing.

Its still behaving the same damn way after all this. I talked to a Ford Master tech; he suggested I adjust out the plunger thing from the brake booster. I will try this tomorrow.

I am running out of stuff to change and adjust. Bummed and frustrated at the same time.

 

The Plunger adjustment worked! I probably had to adjust it out about 1 1/2 turns and wah lah... It surprised me by working. I actually tried backing it off a 1/4 turn to make sure I didnt adjust it to much and it went back to pumping. Sort if surprised how sensitive it is to precise adjustment.

On thing I believe I realized from all this is that the Plunger slightly engages the master cylinder even when the brakes are not applied; the effect seems to hold a certain amount of pressure in the system.

Thanks for all the responses.

 

Glad it worked out for you

There shouldn't be any engagement at "rest". The push rod needs a slight amount of "free play" as written below in the Factory Manual.

 

OldStyle is absolutely correct. If the compensating ports are not cleared and pressure is retained within a few miles you are going to have locked brakes.

 

Perhaps not then... I drove some ten miles or so, both freeway and in town. No issues.

 

Hello all and please forgive me for barging in as newbe into the middle of thread. . .

Bottom line; I have 79 F350 4 X 4 Super cab that has never stopped worth a darn since I've owned it (1997). In short everything has been replaced except the valve in this thread and all to no avail. This truck will not lock the wheels no matter how hard the pedal is mashed to the floor much less slow down if loaded with 1/2ton. It hardly locks them on the ice and has never skidded the tires even in the summer rain! Engine is 460 which is fresh and pulls good vacuum. In short the pedal goes hard but the truck just keeps moving. It's as if it has no vacuum assist but the booster is new and passes all FoMoCo tests.

So my question is this; has anyone ever gained more brake pressure to the drums by changing this combination valve/metering block/Proportioning Valve?

To date and very recently:
Master Cylinder standard bore 1.0625"
Vacuum booster (adjusted plunger for 1/64" gap to MC)
Calipers rebuilt
Rotors trued
Front brake lines
Brake pads

Rear:
Drums
Shoes
Hardware kit
Self adjusters (which work and lock the star wheel)
Wheel cylinders (1-1/16")
Rear brake hose
Adjusted to drag light to moderate.

So, I have never pulled the metering valve bleeder rod out during bleeding. The pedal is not spongy at all. Its solid but it just doesn't stop. The shoes are adjusted out, E brake works great but with the booster bled off, I can have someone mash the pedal with both feet and I can turn the drums by hand. Yes, by hand! During bleeding of the rears the volume is great. But it just doesn't build any pressure to engage the drums in manner that does anything to slow this thing down.

I'll probably keel over backwards if pulling the plunger out fixed it or even replacing it. But I have little hope after all this time and effort that this will fix it. Could the proportioning valve stick in a manor that dropped the pressure down so low as to render the rear brakes useless? And even if the rear brakes are not helping an empty truck should stop with these rotors but it hardly slows it down. In fact it looks as though its headed toward glazing a set of pads over as they just are not getting pressure to make heat and stop this thing.

So I'm headed towards a Dual Diaphram Booster as I don't understand why neither end builds pressure.

Looking for advice so let me know what questions you have. I'll happily start a new thread but my focus is to rule out the combination valve.

Thanks in advance!

 

If you "can have someone mash the pedal with both feet and I can turn the drums by hand", then you have no hydraulic pressure going to the rear brakes. Have you been able to bleed the rear brake though the wheel cylinders?

The valve is a combination valve as you stated, doing many different things.

Reviewing:

One part of it is a pressure differential valve. When there is a difference of more then ~50 psi between the front and rear brakes the valve will shift and shut off flow to the lower pressure circuit as it's design is to prevent excessive loss of brake fluid. If the front brake pressure is low, then the combination valve also will "kick out" the rear proportioning of the hydraulic pressure going to the rear brakes, giving full pressure capability to the rear brake during the front brake failure.

If you are going through all those frustrations, I would just change the valve for the ~$50 or so they cost, especially if you are not getting good flow during bleeding.

 

Complete Brake Upgrade for 1978 - 1979 Ford Broncos .: Articles .html

 

fmtrvt, thanks!
Both ends /all four corners flow great volume during bleeding and as I "kinda stated" until this thread I never knew the combo valve needed its hand held to bleed the brakes. Surprisingly after working on rigs of all kinds had never encountered this until I read it here.

For the higher GVW it says to push it in which I have to admit looking at the pictures I can't what it does. Perhaps divert front fluid to the rears but there is no clear path for that???

Anyway, I have never tried playing it as I always thought it took care of itself and simply activated the light should a differential pressure be excessive from a internal/external leak etc. so I have that to try but like I said, the front pads are nearly glazing over from lack of pressure as if neither end builds pressure and I'm at a loss. Its why I replaced the drums and turned the rotors as I wanted them "true" so run out didn't push the pucks and wheel cylinders in. But unlike the other case, pumping the brakes changes nothing! It doesn't build pressure like a leaky bad MC might it just kids hard.

Anyone ever had a need to run the dual booster? It looks like I could gain some mechanical advantage as it does have more surface area.

I'm just thinking out loud.

 

Thanks!
Just a few lines into this and I think this is where I should have went. . .

"The next part of the system is the vacuum booster. I'm sure you found out by now, the OEM 11" vacuum booster is simply too weak to assist in pressurizing the hydraulic system. The '78-'79 Ford F350 tandem booster (dual 9 1/4" diapragm) is a bolt in retrofit with twice the power output."

I did the math and you get approximately 50sq/in of surface area to apply vacuum too. Damn - I wish I would have went with that after doing all the research!!!!!