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I have a '73 F350, originally had disk brakes in front and drum brakes in rear. I have added disk brakes to the rear so I now have disk/disk. Based on this change I have replaced the original pressure differential (C8TZ 2B257-G) with a VL3359 Combo Valve from Master Power Brakes which is specifically designed for this application.
My question is related to the brake pressure warning switch. The original factory wiring to the pigtail has 2 wires and the plug at the valve has two pins. The new Master Power Brakes Combo Valve switch has a single pin. I purchased the matching pigtail from the manufacturer which is a single pin and has one wire.
Does anyone have and ideas or suggestions for how to wire in a single wire pigtail so it works with the dash light? Is one of the two existing wires a ground which could be abandoned or possible grounded to the frame?
Well here is what a 78/79 diagram says about the wiring. Did/does yours have the light/proof test upon start up and run that brake light a a few sec?
Does Master Power Brakes have any input on their way of wiring it up? Sorry not much help.
Looking at the diagrams above, it looks like the truck's wiring to the switch consists of two wires: One wire which comes from the ignition switch, and one wire which goes to the gauge cluster/warning lamp. That makes me think that the switch is on the positive side, and the lightbulb gets its ground from the cluster. The wires appear to both be the same color combination too.
Your one-wire switch on the new pressure differential valve sounds like it can only complete a ground. The switch grounds to your transmission, and the wire terminal is probably normally dead, and is connected to ground only when the pressure drops.
I'm not 100% certain of that, but I strongly suspect it.
If it is that way, then your warning light will need a constant source of ignition power so the PDV switch can turn on the light by completing the ground circuit. In this case, you would lose your warning light prove-out feature, and the light would only work if there's a loss of pressure. You would need a separate switch on the ground side to test the bulb.
I think the way a stock untouched truck does it, is that when you turn the key and crank the starter, the ignition switch sends 12V+ to the cluster, bypassing the PDV switch, and lights the light while you crank, so you can be sure the bulb is good.
Thank you for the diagrams. It is a frame-off resto and I am a little ways off from startup. Master Power Brakes said to try hooking to each wire separately to see if it will work. Not a solid plan!
Originally Posted by 77&79F250
Well here is what a 78/79 diagram says about the wiring. Did/does yours have the light/proof test upon start up and run that brake light a a few sec?
Does Master Power Brakes have any input on their way of wiring it up? Sorry not much help.
STLRyan.... 6 by 8 advice/input is something to definitely consider. The diagrams I posted are for a 78 NOT a 73 and you might have some wiring differences??? I am not sure is a 73 has the brake light proof test upon startup.
6 by 8 I read it, I appreciate it, I even copied it for my FTE library later. Good stuff thanks.
NOT BS...that is how you have to wire it if you want to retain the prove-out function. The ignition switch does NOT provide power...it simply grounds that circuit during cranking to turn the brake light on (simulating a pressure variance, which also simply provides ground for the bulb circuit).
NOT BS...that is how you have to wire it if you want to retain the prove-out function. The ignition switch does NOT provide power...it simply grounds that circuit during cranking to turn the brake light on (simulating a pressure variance, which also simply provides ground for the bulb circuit).
So I wrote the BS? Makes sense because I wouldn't think they'd run a 12V+ wire down to the PDV. But the schematic diagrams made me think it was positive.
So, if I've got this right, my non BS comments, which I thought were BS, are in fact not BS.
And my BS comment which I thought was non BS, is in fact BS.
Ok, got it, thank you Gentz.
On a serious note, after I read Meangreen's 1st post, only then did I look at the wiring diagram, which by the way looks to be the same as a 1973 diagram in my Haynes book.
I then thought, as Meangreen did, that the wires were live going to the switch.
Seeing as I've never wired a 2 wired switch, I thought it best to call my comments BS to play it safe.
I hate making comments based on assumptions or on something I've read.
If Meangreen did in fact write BS, it was good BS. LOL
A suggestion for STLRyan :
If you have the time, money, and inclination, and seeing as you are in the middle of a restoration :
Give the combination valve away.
Connect the front brakes via a T connector.
Connect a proportioning valve in the rear circuit. (Although it may actually not be needed.)
Fit an aluminum/plastic modern MC
Run the two wires that went to the OE combination valve to the reed switch built into the new MC
The benefits ?
1 You'll feel the improved braking without the combination valve, and the nose will not dive under hard braking.
2 If there is a brake fluid leak, the warning light will come on when the MC is still 50% full.
(I had two slow leaks in the rear drum cylinders over the years, and I only found out when the brake pedal went to the floor !
Luckily both times I had just jumped into the truck ready to drive off. The rear MC compartment was empty and no visual external leak at the rear.
You don't have cylinders anymore, but a leak is a leak regardless.)
3 A reguar visual check of the MC level is quick 'n easy.
Always open to learning a better way (but so proud of my brake line routing, hate to redo!).Do you think I will see the nose dive issue with combo valve and disk/disk? I thought it was ok on disk/disk to skip the delay. Maybe your saying I need proportioning to favor the rear?
I'll tell you what I did, and why I did it, and then you can decide if you want to make any changes.
Starting with the MC, I don't like the OE one, how it looks and how it opens, and I didn't like the fact that I could be coasting down a steep hill only to find I had no brakes without any prior warning.
So ! Safety is paramount, and that's why I fitted an aluminum/plastic modern MC, and wired it up to warn me of brake fluid loss. (as opposed to a pressure difference warning, which DOES NOT work as a useful warning anyway)
It makes NO difference at all to the braking feel and efficiency.
Your existing OE MC should have a 1.0625'' bore, and a 1995 Ranger MC would be used to replace it :
The larger bore moves more volume and offers less pressure.
My pedal travel was 'corrected', and the extra leg pressure I needed to use was minimal, so I think it could work on your truck no problem. And yes, it looks a lot better than the Ranger one !
If you decide to change the MC to an aluminum/plastic one, before bench bleeding, booster rod gap checking, fitting and bleeding, connect the two existing wires to the two outer pins in the reed valve, turn on the ignition, and the red light should come on.
The reed valve switch on the F150 did not work when I got it, so I went to an electronics shop and bought another 'little brown thing' as per the picture :
The black box slides out easily from underneath the MC.
For illustration, I used these for the connections :
Over the years I replaced discs, pads, drums, shoes, MC, booster, rubber tubes, basically everything except the combination valve (CV).
And each time, the brakes felt the same as before, OK but not great.
I thought about it one day, and realized I'd never changed the CV, so I got a new one and swapped it in.
No change whatsoever.
After that, I simply assumed that a 1970's F series truck had sh$t brakes.
Anyway, a few years later, I decided to change the rear drums to discs, for two reasons, I was curious, and, I hated having to remove the drive shaft, bearings, and, and, and, to replace a leaking cylinder.
I decided to remove the CV prior to doing the rear disc conversion, although I can't remember why I decided to do that.
Around about this time I bought a flaring tool, the cost of which, was probably covered by the savings in not having to pay a shop to do things.
I bought it because I knew I was going to be making lots of connections and changes (like the MC), and I wanted the convenience of doing it all at my leisure in my own time.
Anyway, I fitted a new rear brake line, set the CV on a shelf, and went for a drive.
F$$k !!!
The braking was brilliant !!??
I returned home, took the CV off the shelf, and put it straight into the bin.
During the drive, I braked hard to see how bad the nose dive would be. Hah! What nose dive ?
In the attached diagram, you'll see a 'metering valve' on the left, and this is supposed to prevent nose dive.
Which reminds me, I shouldn't have mentioned it earlier seeing as you have disc/disc. Duh !
The proportioning valve is on the far right of the picture, and without the CV in place, I also expected rear wheel lock up under hard braking. Nope !
How do we know that those valves actually work, or if they need calibrating ? We don't.
I fitted a proportioning valve in the rear circuit when I converted to discs, but I've never had to use it. (I.E. the rears still get 100% unrestricted supply.)
If you do decide to remove the CV, I'd suggest waiting until you are driving around with it, then remove it, so that you can experience the before/after effect.
Then decide if you need a proportioning valve or not.
I used the following to connect the front MC input to the two front outputs :
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