Brake Pressure Differential Valve

[venturasurf]

Okay, I've been reading into this a lot and I want to see what you guys think.

The differential valve appears to me to only turn on the "brake warning light" if there is a loss of pressure on one side of the brake system. I initially thought due to the comments of others that it also limited brake pressure to one side as well, in effect "closing off" the low pressure side.

Interesting information is here: http://forums.vintage-mustang.com/vinta ... lve-2.html

Notably "switch OFF"



and "switch ON"



(Posted by PHW over at Vintage Mustang)

Basically, someone cross-sectioned one of the valves and showed how its only purpose is to light the indicator light.

If you look at the diagrams here:



(posted by ultraranger here at FORDification), it shows in the shop manual how the valve operates.

The dual-reservoir master cylinder would the be safety feature in case pressure was lost on one side. One reservoir would drain out while the other remained full. If this differential valve "locked" off one side, it would completely jam the master cylinder by causing infinite pressure on one reservoir when depressing the brakes.

So, lets assume I want to be somewhat unsafe and extremely freaking lazy. If I open the bleeders and find that I can get pressure at all 4 slave cylinders, all 4 of the brakes operate, there is fluid in the master cylinder and the system is properly bled - AND the warning indicator light is still illuminated, the truck is theoretically in a safe operating state. The downside to this is that if I actually end up getting a dangerous brake problem I would be none-the-wiser because my light is always on anyway. I'm not actually going to be this lazy, I am just theorizing.

Thanks in advance for any input.

[ultraranger]

I don't claim to be a Ford brake expert, as I too continually learn new things about their brake systems. On the other side of that, I do have a lot of hands-on and practical experience with Ford brake systems with front disc brake conversions as well as all-wheel disc brake conversions. The following examples are on early Mustangs but, the overall basic operating principles of the brake systems are the same for the trucks.

In 1999, I designed a set of adapter brackets that would adapt '82-'87 Lincoln Continental/'84-'86 Mustang SVO/'84-'90 Lincoln Mk VII rear discs and calipers to an 8-inch or 9-inch rear end housing for the '65-'73 Mustangs and related Fords. I began producing the adapter brackets for sale to the public in 2002-2014. I recently discontinued production of these brackets because wrecking yard disc brake donor parts for these models has become hard to find.

Mk VII rear discs adapted to a small axle bearing 1959 Ford 9-inch rear on a '65 Mustang.



In late 2003/early 2004, I designed another set of brackets to adapt 1994-2004 SN95 V6/GT rear discs and calipers to the 8-inch and 9-inch rear ends of '65-'73 Mustangs and related Fords. Shortly after this, I designed the rear disc brake brackets to adapt '94-'04 Cobra rear discs. The following photos represent a small portion of the experience I've had in dealing with and modifying/adapting Ford brake systems.

SN95 V6 Mustang rear discs adapted to an 8-inch rear on a '65 Mustang fastback.



1994-2004 Cobra Mustang rear disc adapted to a small axle bearing 9-inch rear end.



Front caliper adapter bracket I designed to put 13" Cobra rotors and twin-piston aluminum PBR calipers on a '68-'73 Mustang/'75-'80 Granada disc brake spindle.







SN95 Cobra Mustang front discs installed on my '68 Mustang.





A 1967 Mustang a guy brought me to convert from 4-lug front drums to 5-lug discs from a '75-'80 Ford Granada. A before shot.



After.






Some information suggests the pressure differential valve doesn't cut off the fluid flow. Some information suggests that it does.

http://www.classicperform.com/ShopTalk/2/ShopTalk-2.htm

In the end, if your brake warning light is on, I suppose it doesn't really matter whether the fluid flow to the failed side is cut off or not. The bottom line is if the light is on, there's a problem with the brake system and it needs to be addressed and corrected.


...in the case of MC terminology, I have also been guilty in the past of referencing it as a "dual" master cylinder. As I mentioned above, I continually learn new things about these brake systems. The MC you will find on any Ford vehicle from 1967 to the present day is technically not a 'dual' MC. It is a 'tandem' MC. The primary and secondary pistons are in line with each other and are contained in the same MC casting.

A 'dual' MC is physically 2 separate MCs, mounted side - by - side, with a balance bar connecting them together. These are primarily found in race cars and not in street cars. The brake pressure bias can be adjusted between the two MCs by the balance bar.

http://www.bicknellracingproducts.com/i ... bar-set-up

[venturasurf]

Thanks for the response and for correcting me on the terminology - I'll refer to it as a tandem master cylinder.

It is my understanding that a tandem master cylinder would have the same problem as well. If one of output lines is shut off, it would cause difficulty moving the MC piston and would lock up the whole brake system. The brake valve mentioned in the link you posted shows a different system than the one I posted previously (as well as being different than the one in my '72) and I can see where that particular system might have a low pressure shutoff function.

I have successfully repaired the brake warning light by bleeding off a bit from the rear brakes, the warning light has gone off. I have not noticed any difference in the brake function on the truck though. Peace of mind feels good though.

[MachMe69]

hey Everyone.....ANyone.........I desperately need to KNOW THAT SN95 PBR Setup manufacturer and/or company.....I am trying to do this conversion, need that Adapter/conversion piece.

any help is BIG

regards,
Jeff

[colnago]

You said, "If I open the bleeders and find that I can get pressure at all 4 slave cylinders, all 4 of the brakes operate, there is fluid in the master cylinder and the system is properly bled - AND the warning indicator light is still illuminated, the truck is theoretically in a safe operating state."

As I understand it, you can't have this situation. If you have a low-pressure side (and only if you have a low-pressure side), the shuttle will slide, and the light will turn on. If everything is working right, then the shuttle is centered, and the light stays off. You can't have a balanced system, if the light is on. If the light is on, either your front brakes aren't getting enough fluid, or your rear brakes aren't getting enough fluid. That's the only reason this switch exists.

Also, there's a specific tool that prevents the shuttle from moving while you bleed the brakes, so that it remains centered and allows fluid to get to all wheels properly. After you bleed the system, you remove the tool, and put in the safety switch.

Joseph

[colnago]

Jeff,

I think LMC Trucks (www.lmctruck.com) sell disc-brake conversion kits for half-ton trucks. I have a 250, so I'm SOL, and on my own.

Joseph

[66f250cs]

Refer to the two pics of the valve cutaway. It the shuttle valve is all the way to the left, the light should be on and with the small diameter at the end of the shuttle, fluid should still travel to all of the brake cylinders. If the shuttle is all the way to the right, same thing; light on and fluid flow. So it looks to me that it is conceivable that you could have fluid to all wheel cylinders but the light would be on. JMHO. The pic of drawing of the valve doesn't show this as clearly.
Ok long story. Bought a 69 F250 a year ago. Brakes were beyond horrible. No lamp in the warning socket. Installed new lamp. Light is now on and stays on. Removed Kelsey Hayes brake controller from cab, also wanted to remove tee into front side of MC to clean up under hood area. Popped top off MC, bone dry in front part of MC. Great, I can remove tee without fluid going everywhere! Removed tee, installed brake line into threaded port. Fill with fluid. Test drive to alignment shop. (The truck was soooo bad going down the road.) At shop, under truck snooping around, the mechanic says, looks like you have a bad wheel cylinder, left rear. Sure enough he's right. Replace wc and brake line (split at wc end). Fill mc, bleed system, test drive. BANG! Now what? Go home snoop around nothing obvious. Pull right side rear. Broken parking brake lever spring and broken e brake cable spring. Finally found a pb lever spring and e cable, installed, reassemble along with new shoes and hardware. Go back to left side and install new shoes and hardware. Test drive. Much better but still marginal. Pull front dual piston calipers. Both sides are frozen. Back to Napa. Get calipers hardware pucks bearings seal etc. install and bleed. Right front caliper leaks at bleed port. Remove and return to NAPA. Install a replacement. Leaks . Install another one. Ok finally stopped leaking. Test drive. Terrific! Brake light still on. The shuttle refused to self center to turn off light, soooo remove and rebuild. Also made a shuttle valve "hold device". Reinstalled rebuilt valve and switch. Light is now off and brakes work very well.
Ok going to digress here. Got new tires for my 86 E350. Before I put them on I thought I'd try them on my 69 to see if they would improve ride/handling. It still had the split rims and bias ply tires; three different manufactures. WOW. What a difference. The truck handled beautifully drove straight, stopped straight, a totally different animal. So anyone wondering if welded steel wheels and radial tires and a brake overhaul is worth it, the answer is "definitely".
Final note re the original post on this thread. Center the valve on your brake system. It's the only thing that tells you at a glance you have some kind of fluid pressure imbalance in your system. Ford provided it to you for a reason.

[colnago]

You're right. I was thinking of older-style MCs, that used a single bowl for the fluid. If you had a leak in the system, you didn't want to keep pumping fluid to that side, or you might pump everything out of the leak. With the valve in the picture, it will still allow flow, so you should at least have partial braking. But since newer MCs have two bowls, the "good side" can't run dry.

Joseph

[ultraranger]

Tandem MCs (split brakes systems) didn't come into existence until 1967. The pressure differential valves also didn't exist prior to 1967 --a single reservoir MC would not have had the safety feature of a pressure differential valve. The line would come off the single reservoir MC and then go to a Tee and then the lines branched off from there but, the Tee had no internal valve function with a single reservoir brake system.

This is a tandem MC and the old drum/drum PDV off my '69 F100 in the photo below. I assembled this setup just to test out what happens when the pressure differential valve spool shifts. The spool inside the valve is shifted toward the ports that supply fluid/pressure to the front brakes. In this setup, brake fluid freely flows from the MC's foward-most port and out the port of the PDV where the line would attach going to the rear brakes.

On the PDV valve ports that would go to the front brakes, no fluid from the MC's rearward-most port comes out of the PDV with the spool shifted. --the light bulb (with the blue plastic cover over it), is hooked up to a 12 volt DC power supply and is lit up, showing that the PDV spool is shifted off-center.

[fijidad]

My understanding from an earlier discussion on this forum was that this valve's job was to apply the rear brakes just before the fronts...not side to side issues. Take a look at this earlier link: http://fordification.com/forum/viewtopi ... 44#p710344

[ultraranger]

The pressure differential valve was first installed on in the Bumpside trucks beginning in 1967. It warns that there's an imbalance between the front and rear brake circuits, if there's a line or brake component failure where brake fluid is lost from the system.

'67-'72 F100s were never available with factory installed front discs. Only '68-'72 F250s/F350s were optionally available with factory front discs. The F250s/F350s with optional front discs used the same pressure differential valves as the drum/drum '67-'72 F100 through F350s. The only difference was the F250s/F350s with factory front discs came with a stand-alone metering valve plumbed into the front disc brake circuit.

The multiple brake valve functions in one valve assembly: Metering (hold-off) to the front discs, pressure differential sensing between the front and rear brakes and proportioning to the rear drums didn't come into being in the Ford trucks until 1973.

[Thipdar]

It's not a "valve".
"Valves" change the direction of the flow of a liquid.
While there may be a change to the flow of brake fluid when the switch is activated, it is incidental and not the primary function of this device.
In this case, a change in the differential pressure is what activates the "switch".
When the switch turns on, it illuminates the warning lamp... but the change to the flow of brake fluid (if there is any change) is of trivial importance.