Low Brake Fluid Sensor Connection 86GT with 93 Cobra Master Cylinder 5 Lug Conversion

[1200]

The float switch in the reservoir is a magnetic switch. It is simple a low fluid level warning to tell you the reservoir is low. It is a simple courtesy feature to alert you to a minor maintenance item. The brakes may still function perfectly normally.

The switch on the proportioning valve is a brake failure warning. Its job is to let you know when there is a dangerous condition that will severely limit your braking capability. These are not different ways of doing the same thing, they have very different purposes. It is entirely possible to have a full fluid reservoir and still have a brake failure condition that would cause the warning light to activate due to pressure differential in the two sides of the system.

Hey guys, sorry for the delay in responding, work is kicking my butt! No time to get to the car, but the weekend is near... back in the game then.

Yeah @BMW Rider, I understand what you are saying.... It is your very point that compels me to attempt to make both of those systems work for my 86.

BTW, don't worry about any criticism, it's all good

I'll post up once I get a chance to spec the wiring...

Thanks for you input though

Dwayne

I have the pigtail plug ends for late fox cars and the SN95 cars. I'm in the process of working the wiring diagram thru for both to see what does what according to wire color codes. Ther are 3 wires on the late Fox cars..... 2 of those wires will for sure connect to my 86 in parallel...that's the plan.

[1200]

If you gutted your proportioning valve as I did, you removed the piston and spring in the center of the proportioning valve, when you removed the piston you took away the perch that the switch attaches to. There is nothing physically left inside the proportioning valve to trigger the switch to alert you to the differential of pressure in the brake system. When you removed that piston you also removed the ability of the valve to do it's job of distributing the fluid at different levels, that's why you need to install a manual adjusting valve in the rear brake line.

The photo you had of the reservoir is the same as what I used, the top 2 pigtails that you have in the photo are the style that I used. I believe that the switch is triggered with a magnet in the float. If you look at the plastic reservoir you will see it has one inlet with two chambers inside, the fluid level will lower if a brake line leaks, but it has a divider inside that will hold enough brake fluid over either the front or rear portion of the master cylinder that you will still have functioning brakes and it will alert you to an issue by triggering the light in your dash.

Thanks @mb757, I will no doubt follow thru with your procedure, but what I was trying to say in my previous post to you was that the PV /Combo when gutted still leaves the Line Pressure Valve (LPV) unaffected. Take a look at the actual 86GT PV / Combo Valve pic below..tell me if you understand what I mean or if you think I'm mistaken..

Note: the LPV is the mechanism on the left side of the Comdo Valve. The Metering Valve (gutted side) of the Combo Valve is on the right. My understanding is: By gutting the (rear brakes) metering side of the CV, there will still be line pressure to the Line Pressure Valve LPV that can in effect still actuate the Line Pressure Switch in the event either front or rear brakes loose pressure.

[1200]

BTW, DOES ANYONE KNOW THE THREAD SIZE AND PITCH OF THE 86GT SWITCH?

I found a tool that is very helpful for bleeding the rear brakes following the PV ; Combo Valve gutting. It locks the LPV in place during the bleeding so it can't shift and shut off either side of the lines. The one I found is said to be for GM cars though and the merchant doesn't know the GM thread specs. and yes, GM uses this same setup.

[Jack Hidley]

1200,

Your understanding in post #27 is correct.

Ford does some unusual things with the combination valves. For instance the nut that plugs the proportioning end of the assembly is not a standard thread pitch and diameter. They may have done a similar thing with the differential pressure switch threads.

[Jack Hidley]

1200,

Your understanding in post #27 is correct.

Ford does some unusual things with the combination valves. For instance the nut that plugs the proportioning end of the assembly is not a standard thread pitch and diameter. They may have done a similar thing with the differential pressure switch threads.

[1200]

1200,

Your understanding in post #27 is correct.

Ford does some unusual things with the combination valves. For instance the nut that plugs the proportioning end of the assembly is not a standard thread pitch and diameter. They may have done a similar thing with the differential pressure switch threads.

Thanks Jack for this confirmation. I'm kinda getting my mind around the design of this whole system. I understand how the Proportioning section of the Combo Valve meters fluid to the rear drums/calipers under braking to prevent rear brake lock-up during heavy braking situations. I also understand the concept within the Differential Pressure Valve side of the PV/CV.

What seems to me to be odd is where Ford stopped using the Differential Pressure Switch on the 87+ cars and opted only for the Reservoir Low Fluid Level Magnetic Switch to warn of any failure. ?? My thoughts are that it would have been better for them to use both LPS and LFLS. Actually, that is my plight.

I want to make both systems work on my car. I have the process pretty much mapped out as to how to make it all work together, except I need one last bit of information about the 86 and earlier pressure switch wiring polarity. My car has the dash out so I can't check it.

I was hoping to get someone else to probe their car to tell me if the circuit is a simple negative loop in and out of the switch or if it's a (pos) to ground at the switch itself? Either scenario, I can make it work, but the wiring will be different depending on this polarity event.

Anyway, thanks a lot for your comments. I really appreciated it.

BTW, Have anyone heard of the tool below that locks the Differential Valve in place during 5-lug conv. brake bleeding? The tool looks like the pic below. I just need one that fits my 86 PV/Combo Valve. I can find them everywhere for the GM cars, but noone that I've spoken to knows for sure if they will fit the threads on the Ford PV/Combo Valve....

[Jack Hidley]

The most common failure mode that the differential pressure switch is going to protect against is when a caliper piston seal or a brake hose bursts. This valve should shut off that brake system immediately and the switch will trigger the light on the dash. The low reservoir switch does essentially the same thing as the switch function, it just does it a little bit later.

To bleed the rear brakes on a new install, only open the bleeder screws a little bit. This will keep relatively high pressure in the rear brake system, which will keep the differential pressure valve from triggering and shutting off flow to the rear. I've never seen that plastic tool for a Ford combination valve.

[1200]

Another piece of the puzzle solved:

Getting ready for my 5 lug conv. brake bleed job, I found that many were having or had issues getting their system bleed down correctly. After a bit of research, I discovered most had not considered using the PV/Combo Valve Bleed Tool to lock the Low Pressure Valve in place. What can happen is: the low pressure valve can shift inside the Combo Valve shutting off one half of the lines. This will make it almost impossible to bleed the brake correctly. Or, it may partially shift and you end up with air bubbles....

I looked everywhere for one of these tools known to fit the 86GT Combo Valve - and I got nowhere! I decide to make one! Below you can see a photo of what I came up with.

Here's the deal: Find a Brake Line Fitting with 3/8-24 thread, this is the sizing for the Pressure Switch on the 86GT Combo Valve. (may be the same on other Fords) Leave a piece of the flared brake line with the bevel in place. I simple cut the fitting off my old lines and left the line stuck in place. You need the line in place because the hole in it is about the same size as the pin prong on the pressure switch that must fit down inside the little hole in the CVbody.

Get a screw small enough to thread into the bevel flare of the old line piece and screw it in tight. Using a dremel tool, cut off the head of the screw leaving a snub of thread sticking out. Use the original pressure switch to measure how long to file the screw piece down to the correct, same length as the switch pin that protrudes out. This is the depth needed to lock the valve in place inside the Combo Valvebody. Depending on the thickness of your screw, you may have to file it down to the right diameter to fit down into the hole inside the CVbody. Once you have test fitted everything, you can then use JB Weld to secure everything together, being sure not to get JBW on the threads of the flare fitting. Once it hardens 24hrs you can fine-tune the fitment. All you need to have happen is for the pin to make contact with the LPV slide inside the Valve Body; then it can't move. GM guys use a plastic tool so this metal one should be fine DO NOT over tighten this tool in the Valvebody, a little pressure will do as it won't leak fluid form this location so long as the seals in the Combo Valve are good. A picture is worth a thousand words. see below.

Hope this helps somebody during their brake bleed. BTW, get a pressure bleeder as well to make the whole job go well. I hear they are a must.

Dwayne

[svono50]

Hmmm, I always vacuum bleed my brakes, so do you really need to lock out the PV/CV when performing a vacuum bleed or is this only necessary when pedal bleeding a system where you are 'pushing' fluid through the system? I have been vacuum bleeding for >20 years now on all kinds of brake systems (non anti-lock and anti-lock) and never had an issue with getting bubbles or incomplete system flush. Sorry for diverting from the OP's topic a bit.

[1200]

Hmmm, I always vacuum bleed my brakes, so do you really need to lock out the PV/CV when performing a vacuum bleed or is this only necessary when pedal bleeding a system where you are 'pushing' fluid through the system? I have been vacuum bleeding for >20 years now on all kinds of brake systems (non anti-lock and anti-lock) and never had an issue with getting bubbles or incomplete system flush. Sorry for diverting from the OP's topic a bit.

@svono50, It sounds as if you've had a great deal of experience bleeding break systems. Thanks for responding.

You're probably correct in your assumption that the sort of difficulty that many have described is in part a result of pedal bleeding in some form. I'd like to think the cause is the lack of back pressure on the LPV at this juncture. And to answer your question: No, you don't need to lock out the LPV during every vacuum bleed, though it wouldn't hurt to have that assurance in place by using the tool if you have it. You may even discover that the seals may be leaking in the LPV when you use it....If fluid leaks there, you have seal issues that need to be repaired. Old fluid crud is most likely the culprit.

However, in this particular case, as described, it is more focused at the complete system change where the PV/Combo valve has been removed and altered during the 5-lug conversion process. Many during this process will change over to stainless steel lines, deployed late-model calipers and the like. The system them will have no fluid pressure in the system at all. It is during this stage that the difficulty talked about in this thread, comes on hard. In addition, the potential to shift the LPV is high, especially if it has been removed earlier and is more free moving in the valve body. The evidence of a tool being manufactured to avert this event is testament to its potential occurrences, on Fords and GM's.

Thanks svono50.

[Jack Hidley]

Ted,

I doubt that vacuum bleeding could ever cause the differential pressure valve to trigger. When applying a vacuum to one end of the brake system, the absolute maximum pressure drop that can occur across the differential pressure valve is one atm (14.7psi). I really doubt that this is enough pressure to move the valve.

With a brake hose failure at one end of the car, there could easily be hundreds of psi across the valve.

[1200]

Hey guys I have not found a thread, on any site, that talks about connecting the low brake fluid sensor wires from the 4 eyed cars to the popular 93 Cobra, 87-93, and SN95 Brake Master Cylinders.

How can I make my low brake fluid senor in the dash work after doing the 5 lug conversion? I am using the 93 Cobra M/C. I can use the SN95 V6 unit but prefer the Cobra 1" bore.

thanks.

SOLVED Thanks to those guys that helped out. I appreciate it

See solution below

Notes:

1. Getting to a concrete solution often involves collaboration of like minds working together. In addition: reading and research, along with a desire to learn.

2. The name "Proportion Valve" (PV) as we know of it is not the correct name for the part as we know it. It is actually the " Brake Pressure Control Valve" or aka "Combination Valve," because of its multi-funtionality. It is a Metering Valve (MV) for fluid to the front brakes, a Proportioning Valve (PV) for fluid to the rear brakes, and it is a Pressure Differential Valve (PDV) to complete continuity in the Pressure Differential Switch (PDS) circuit thar lights up the dash Dual Brake Warning Light under brake system fault or ignition contact during Key-On operations.

3. There are no wired circuit on the Fow and SN95 cars the relate to brake pad wear sensing. Only fluid level / pressure sensing circuits are in play.

4. The term Dual Warning Brake Switch is actually correct for both the LBFL switch and the older line- PDS as the duality is in the parking brake warning and the fluid level / line pressure warning operations.

5. 1987 and up Plastic Reservoirs with 3 wire circuits containing the Magnetic Switch at the bowl performs the same task operation process as does the 1986 & earlier cars in their LPV circuits - A mechanical ground circuit that actuates the dash warning light upon ignition "start" contact and during maluction or fault events. (SEE DIAGRAM BELOW)

6. Read full thread if you want to know about compatibility.

SOLUTION DIAGRAM:

[mustangwolf]

TOUCHDOWN!!!!! as one of our other fellow stanger would say.... This is the answer I've been waiting for confirmation on. I thought this would be the working scenario for the conversion, but no one could or would confirm it. Thank you @mb757 you win the PRIZE!

Now just to add an additional question to you. I pulled a couple reservoir switches from the boneyard. I believe 2 of the samples in my photo is the p/n that you listed.... If not exact, I'm sure they will do the same job as they are from the non-hydroboost cars.


I am still curious as to how these plastic bowl reservoirs transfer their low fluid signal down to the senor switch? There seems to be no physical connection between the bowl and the contact point to complete the circuit.... Maybe it's magnetic???

Attachment 109797

This is my new Reservoir sold as being for the 1993 Cobra Master Cylinder. It looks just like the later model Fox cars.

Attachment 109799


I'LL REPORT BACK WHEN IT'S ALL HOOKED UP AND DONE!

Any idea where to get one of the three prong connectors that plug into the master cylinder? I would like to wire my 93 cobra master cylinder low fluid level indication into my '83 wiring harness. Is there a specific name for that type connector?

[Jack Hidley]

Look in the Motorcraft pigtail book.

[stangPlus2Birds]

Fwiw, the best I could find is an expensive ebay listing for ~$45.

https://www.ebay.ca/itm/For-1995-199...-/392394932838

For 1995-1998 Ford Mustang Brake Fluid Level Sensor Connector Motorcraft 35468SB
1996 1997
Quantity: 4 available / 3 sold
Price: US $45.25

[massacre]

This also a good subject for those doing a hydro boost swap. Hydro boost master cylinders have the magnetic sensor.
Thanks for posting this! Very helpful

[qikgts]

Bumping this thread to add a link. It's for a Ford specific "Pressure Differential Switch Lock Tool", a premade version what user "1200" showed us all how to make in post #33.

https://www.musclecarresearch.com/brake-valve-tool