Some people say the PDV only turns on the brake warning light and does nothing else. Some say it turns on the warning light and blocks off the port in the valve to the failed (ruptured) side of the brake system.

At the time, I wasn't completely sure what it did or didn't do so, being the inquisitive type, I took a PDV that I knew was shifted off-center, mounted a MC to a pedal support, ran two short lines between the MC and the PDV and I even wired the pressure switch to a 12 volt power source and I connected a 12v light bulb to it to simulate the brake warning light on the dash (shown lit up under the blue plastic cover).

I started pouring brake fluid into the two MC reservoirs. After a few seconds, brake fluid started running out the secondary side of the PDV but, no brake fluid came out the port on the primary side of the PDV.




If you'll insert one of these pressure differential valve lock tools, prior to bleeding the brakes, you won't be fighting a shifted valve spool.

 

After all the input I've received from all of you about the brake pressure differential valve, I have decided to abandon my search for a valve.

Since the drum/drum brakes don't need it to function properly with the new dual chamber master cylinder, and since my truck (a 65 F250) never had one anyway, I'm not going to try to install one. I don't want to change my ignition switch to a later year type, and leaving out the valve will make the new m/c install a lot simpler.

Thanks for all your suggestions and offers to find me a valve.

 

.....of course, the reason your truck didn't originally have a PDV is because it was built prior to '67 and it had a single pot, suicide, MC on it.

I don't know exactly how the circuit would be wired into a pre-'67, since I've never owned a Ford truck that far back. My '68 Mustang and my '69 F100 already had the wiring and components in place. I don't think it would be that difficult to do though.

I suppose it could simply be wire to a switched hot when the key is on. As long as the spool is centered, the warning light would remain off. The instant, when/if, the spool ever shifted, it would complete a circuit path to ground through the valve body, through the valve mounting bracket, through the chassis and back to the warning light, at which point the warning light would come on.

It would just operate like the brake warning light on the '67 models, which is that it wouldn't momentarily light up when you turn the key to the "Start" position. It would only light up in the event of a failure.

 

I hear what you are saying, and I believed that as well at one point, that it shut off the leaking side. Look at the cutaway in the shop manual. The shuttle has a smaller diameter on the end. It may lessen the flow, but it does not turn it off. If it did, then the front brakes would also be disabled if any fluid was left in the reservoir of the leaking side because it would stop the forward progress of the piston in the MC, as the shut off side represents a hard stop to fluid flow.

 

Steve this has got me thinking. When I turn my key to ON, the ALT and OIL idiot lights come on, and go off when I start the truck. Isn't that a "Prove Out" circuit like you described for the valve warning light? Do you think I could possibly tap into that circuit for the warning light?

 

As I've said, I don't know the particulars to make this work on a pre-'67 model although, I don't think it would be that hard to at least make it function as the '67s did --might not be that hard to make it function like the '68-up models either. It just depends how the '66-earlier ignition switch is configured or, whether a '68-later ignition switch could be installed and how the '66-earlier wiring harness to the ignition switch is configured.

You would need to compare a '65 wiring diagram to a '67/'68 wiring diagram. Fordification.com is dedicated only to '67-'72 year model Bumps. There are lots of wiring diagrams available for viewing on that site.

If you have a Ford Truck Shop Manual for you're truck, it'll include the wiring diagrams for it.

 

I have the 65 Shop Manuals. I'll check the wiring in there and look for the bump side schematics on Fordification. I didn't know that site was dedicated to Bumps. I guess I could put a warning light below the dash. I don't want to drill any holes in it. Maybe make it look right next to the aftermarket gauges the PO put in.

Since the valve was used for possibly other than just a warning light, it might be worth the extra effort to put the whole thing in. At any rate, we'd all maybe learn something. That's one thing I really like about old trucks... and FTE. I'm always learning something.

 

The pressure equalization valve is not a valve in the sense that it controls the flow or pressure of brake fluid anywhere in the system. Rather, it was a hydraulic actuater for the switch that simply indicated a pressure differential situation that could potentially be hazardous, such as one reservoir out of fluid. The idea behind them was the concern that 2 reservoirs, one for the back brakes and one for the front brakes, could mask a failure or problem in one of the reservoir's fluid circuits. The valve/switch is not critical or perhaps even necessary for safe operation of the drum/drum system. Drum/disc systems may need such a warning.

 

Well, imagine my disappointment when I checked the 1965 Ford Truck Shop Manual reprint I have... 3 1/8" thick in 3 volumes and 2 books. There are a grand total of 4 circuit diagrams in these 3 volumes: the "3-speed air torsion spring shift circuit" in Volume 1; the "typical conventional and typical transistor ignition system circuits" and "starting control circuit test" in Volume 2. No truck wiring diagrams. Absolutely worthless for troubleshooting wiring.

But I also have the 12-page 1965 Ford Truck F-100 to F-750 Wiring Diagram Manual from Jim Osborn Reproductions and the 18" x 24" color, laminated print from classiccarwiring.com.

And I said all that to say this: using the last two sets of diagrams, I may be able to figure out how to wire in the switch on the PDV like on a '67. I have no idea how to make a Prove Out circuit.

 

The only Slick I've ever worked on was several years ago when I installed a tandem MC and did the plumbing work on a friend's '65 F100 short bed. He had converted to DJM drop beams and front discs from a '78 F150. Looking back, I wish I had installed a PDV and would know the solution to this but, I installed the components he provided me with.

 

Steve (ultraranger) has made some good points about why Ford must have spent the money to put these PDVs in the trucks starting in '67 with the first dual chamber master cylinders. The dual m/c's were probably because they had to, by law. But the valve was probably a safety measure.

So, I'm still looking for a valve if anyone is still willing to find one for me. We only have one local junkyard, and they don't have any 67 - 72 trucks.

Thanks in advance for your help.

 

I have a valve from a 68 f250 4x4 drum/drum truck if you want. Appears to say C8TA-2B257-R but it’s hard to make out...?

 

Thanks, but I think that's not the correct valve for my truck. I don't know what the difference is, but the 4x4 part is different from the 4x2 part number C8TZ 2B257-D. And I think your part number should be C8TZ 2B257-C.



Maybe Steve (ultraranger) or Bill (NumberDummy) can tell us the difference. If they say it will work in my 4x2 all drum truck, I'll PM you for details on shipping, etc...

 

The difference has to do with the internal port size in the seats where the brake lines connect. The pressure needs to be equal or the shuttle will shift and light the warning on the dash. Different brake arrangements (different size brake cylinders, Dr/Dr, Disc/Dr, different sized brakes, etc) will have specific distribution blocks as a result.
Other changes happen through the model year and new revisions will replace old ones. The 'R' version may well be a later revision of the 'C.'
Lot of effort for an annoying light. 0.02.

 

A question about the plumbing here.

I can see the in and out ports obviously.

But what is the line on the left side of the valve in the pic? Can that be used for connecting a brake light switch?