Still chasing this problem.

What it is:

79F250 4x4
swapped in manual pedal assembly from ?
dual diaphram booster swapped in with matching MC
all brakes lines,hoses are brand new
prop valve is original

Before manual swap, truck stopped great!!! Running 37's(really,really good)

The Problem...no brakes(pedal to the floor). My booster rod was too short to reach hole in new pedal. Rather than lengthen rod,I added material to the front of the pedal. IIRC it was 1.00 forward. I bled the brakes just to be sure and no air was trapped.

If I adjust the booster rod(engine side) under the hood I can get brakes but it seems there is a very fine line between pedal to the floor and the front brakes dragging.

Should I have lengthen the rod rather than mod the pedal? I just cant wrap my brain around that but I aint no engineer. I'm still pushing the rod forward the same distance...right?

Am I adjusting the boost rod(under hood) too much at a time?

Could the prop valve suddenly have gone bad?

Gettin purdy dang agravating.

 

What (specific) booster and what (specific) brake valve are you using? Clear pictures of each posted here would also help clarify exactly what you have.

Also, is the Brake Warning light lit on the dash?

 

Booster is dual diaphram A-1 cardone #50-9337 with matching MC. I found the upgrade info here somewhere. Booster is for 79 F350.

Prop valve is original equipment. From the symptoms I really dont think its the valve but my best thinking got me here.

I'll have to get some better pics,

 

I suspect your primary problem is the use of a revised pedal ratio booster (booster input rod curves downward on the eyelet end) being used with a non-revised ratio brake pedal/pedal hanger. Without the correct brake pedal and pedal hanger for that booster, your pedal will not have much forward travel before it meets the floor and you run out of brake application via the pedal.

If you install this booster, with a straight input rod, your low/pedal to the floor problem will likely go away. (This is the exact same booster (for a '75 F350) I installed in my '69 F100.

BrakeBest Brakes 54-73112 - Power Brake Booster | O'Reilly Auto Parts

 

I did some research with my parts store and I bought both and returned one. Betting I have the correct booster as my rod is straight. We found I had purchased both part numbers.

Pretty sure my pedals are from highboy 4x4 truck. I know ND said there are a bunch of diff part numbers for the pedals.

I'll disconnect the booster from the pedal and see how much inward travel I have vs connected. Dont know what else to try.

 

I remember having this kind of dilemma when I did this swap a few years ago. I wanted that dual diaphragm booster when I converted to a hp d60. I remember the rod being the issue. I think all I did was measure how much stuck out of my power drum booster. Napa helped find the one that was closest. I have the right pedal height but they always seemed kinda soft and the brake light is on so I guess the prop valve is bad. Just something I been putting off. But I was able to use my highboy pedal with the newer stuff and all is well in that area.


Also I'm sure my rod is straight. (Haha)

 

Hey Mark, look on the brake pedal for an engineering number. If you find it, post it and I'll see if I can cross it over and I'll let you know what it came off of.

 

Hmmm, now I'm confused. You mentioned having a brake booster under part number A 1 Cardone 50-9337. That part number goes back to this revised pedal ratio dual diaphragm brake booster with a remanufactured MC, as shown in the photo immediately below.




....but, you're saying the booster you have actually has a straight input rod (?).

If this is the case, the only things I can think of that would make the pedal hit the floor are; air in the brake system, rear drum brake shoes aren't adjusted out (if you still have drums in the rear of the truck), master cylinder is defective (not overly unusual at all with a remanufactured MC) or, the booster is defective.

Did your truck have manual brakes before you added the booster?

 

If your brake warning light is on, it isn't telling you the brake valve is defective. It's telling you that either you have or, have had, a brake component failure in the system or, you bled the brakes out and tripped the pressure differential spool inside the brake valve.

The pressure differential valve spool has hydraulic pressure applied across both ends of the spool. Pressure is applied across the primary end of the spool by the primary side of the MC (port closest to the firewall). The secondary end of the pressure differential spool has pressure applied across it by the secondary side of the MC (the port closest to the radiator).

Under normal circumstances, pressure coming out of both the primary and secondary ports of the MC will be equal. This equal pressure keeps the differential valve spool centered inside the brake valve body. With the spool centered, the contacts of the brake warning switch will remain open and the warning light on the dash will not be lit up.

If there's a brake component failure in the system that allows brake fluid to escape, fluid pressure on that brake circuit will drop. This will make one brake circuit have low pressure and the good circuit will have high pressure. This imbalance in pressures will cause the good side to apply more force on the valve spool and shift it over to the low side circuit within the brake valve body.

When the pressure differential valve spool shifts over, the plunger of the brake warning switch rides up a 'ramp' on the spool. This closes the contacts on the warning switch. This completes an electrical path to ground, through the body of the brake valve, and turns on the brake warning light.

Other than a brake component failure, that results in a loss of brake fluid, the only other thing that can trip the brake warning light (make the pressure differential valve spool shift) is when you're bleeding the brakes out --one circuit will have a lower pressure than the other.

This is a pressure differential valve you would find on a '67-'72 F100 with all wheel drums, '68-'72 F250/F350 with optional front disc brakes or, a Dentside 4x4 with 4-wheel drums. This shows the inner workings of how the pressure differential valve operates.




These are two examples of disc/drum brake valves you would commonly find on many Dentside trucks. Beginning in 1973, Ford disc/drum brake valves on most trucks contained 3 different brake functions within the same brake valve body assembly: a metering valve to the front (primary) brake circuit, a proportioning valve to the rear (secondary) brake circuit and a pressure differential valve in the middle of the brake valve assembly, between the primary and secondary brake circuits.




Here is the Ford procedure, on how to recenter the pressure differential valve spool, to turn the brake warning light off.

 

Mike, I'll check today.

Ultraranger...

warning light is off and connected to valve.

I have the same booster that you installed.

It is not a manual brake truck.

I believe the pedal would be hard to push at the top of the stroke if the booster was bad. You got me wondering about the rear brakes though. Truck was in the shop last winter getting a gear swap. Wonder if something happened and the rears were never readjusted? Odd...truck brakes were fine on the drive home. I'll doublecheck.

I wouldn't say the pedal just drops to floor. It has a very small amount of resistance but does go all the way down. I think this is just a result of adjusting the booster on the engine side though.

Thanks for the help everyone!!!!!

 

Pardon me for butting in, did you bench blead the M/C to make sure there is no air?

 

The booster output rod adjustment could also be a problem with a long pedal travel. If the booster output rod is adjusted too far out, it will always be partially applying the brakes --even when you aren't pressing down on the brake pedal. This will cause the brakes to drag, get hot and can cause them to lock up.

If the booster output rod is not adjusted far enough out, it can cause a long travel of the pedal, before you start having any significant brake action.

Here is a procedure on how to adjust the booster output rod to the MC.

http://www.maximummotorsports.com/as...es/MMBAK-8.pdf

I would definitely check the adjustment of the rear brake shoes and the booster output rod setting. One or both of these could very well be the problem with the brakes.

 

Mike...# D7TA LA /PWR BRAKE....betting 76 F250 4x4

So...the rear brakes weren't even close.Miles away. Wierd how I didn't notice anything coming home from the shop. Anyhow, kinda tough to feel drag with 44's so I pulled them off to adjust. However I ran out of time to check out the pedals.

Sunday I'll readjust outer boost rod as I had a bit of front brake drag when I last worked on it.

MC was bench bled along time ago.

Thanks very much for everyone's help with suggestions, diagrams etc.. Much appreciated!!!!

 

Hey Mark, that one has several applications:
77 F100 manual trans w/ power brakes
78-79 F100 w/ power brakes w/o cruise before s/n DJ0,001
77 F150 w/ manual trans w/ power brakes
78-79 F150 w/ power brakes w/o cruise before s/n DJ0,001
77 F250 w/ manual trans and power brakes
78-79 F250 w/ power brakes w/o cruise before s/n DJ0,001
77 F350 w manual trans and power brakes
78-79 F350 w/ power brakes w/o cruise before (you guessed it) s/n DJ0,001

 

Hmm, that's weird as I dont remember those trucks passing through that was manual trans. Wonder how I came across that one?

Thanks Mike