I have a 1994 Aerostar XLT with the 4.0L and rear ABS. Next to the master cylinder there is a distribution block (proportioning?) that has a switch on top that plugs into the harness. The switch is actually mechanical and the contacts are in the harness. Three pieces: plastic housing, spring, and piston for contacting the harness. Well, the thing leaks and when is leaks enough, the little switch turns on the rear abs light and then the brake light if the fluid gets even lower. Junk yard parts seem to fail quickly and the oem part is obsolete and way too expensive.

Would it be ok to just put a brass plug in the hole and jumper the harness to keep the NC switch closed all the time? For me, it seems a little silly to have the part that leaks telling me I have a leak. I have been playing with this for a couple of years now and am tired of all the trouble. The leak is not terrible, but enough to where I am used to checking the fluid a couple of times a week so I am on top of the system anyway. Thought, ideas, suggestions. Thanks

 

There are two switches on the MC. One is for the reservoir level, and the other detects the position of the shuttle valve. If fluid is leaking on the shuttle valve switch causing it to short, it means your MC has an internal fluid leak, and should be replaced (or rebuilt with new seals, if possible). What's happening is the pressurized fluid that is supposed to be directed to the caliper or master cylinder is leaking past internal seals of the proportioning valve. The leak will only get worse, which could result in loss of brakes at some point. Replacing that switch with a plug is not going to fix the leakage problem.

 

Are you saying that there is never supposed to be brake fluid where the switch is located? It is a mechanical switch, not a pressure transducer. From a historical perspective, the original part leaked and I replaced it. No leaks for about five years and then it starts to leak again. Unwilling to pay for a suspect part again, I picked up a couple a the junk yard and installed one of those. No leaks for about 8-10 months and then leaking again. Put another junk yard switch in and no leaks for about six months. If I look up the original part purchase, I believe I will be at about 8 years total on the problem. If the leak is only going to get worse, would it not have failed within 8 years? Understanding that it is a mechanical switch, I would assume since the fluid is much more reliable and incompressible than than air, there is supposed to be fluid behind that switch at all times. If it is air actuated, shame on the DE on that project and he should go back to school. I will be visiting with my Ford dealer buddy next week and will go over this with him. He was not available this weekend. I will let you know what he says. Thanks

 

If it's the sensor screwed into the front of the MC/prop valve, no, there should be no fluid at all behind it.

Are you referring to the ABS control valve, which is mounted on the inside fender panel below, separate from the MC/prop valve? Maybe your 94 version is different from my 90 model, but on mine, the connector is for providing power to the 2 solenoids that work the ABS valve; they don't look like you described. I'm not sure what piston contacts the harness.

Can you post a picture?

 

I don't know how to post pictures here. But, I did find this picture on the web. RacePages.com you look at the lower left of the picture, you will see the aluminum part with the two hardlines going into it. Between the two lines is the switch. Black plastic part. I have figured that it is normally closed since when I unplug it the light comes on. When the pressure is low, the piston is withdrawn by the internal spring and the light come on. I think the 90 model is a little different, but I am not sure, I see so many in the junk yard when I go. Look at the picture and let me know what you think.

 

The lower block is definitely the proportioning valve. I didn't realize they went to separates in 1994. Both my 1990 and 1991 models used integrated MC/prop valves.

That switch in the middle is for the shuttle valve, and there really should not be any fluid behind it. The switch is not sealed, nor is it designed to hold any fluid pressure. The switch should have a brass plunger that goes into a gap in the middle of the valve. The valve is kept in its middle position by equal pressures between the front and rear brake circuits. If one side leaks, the valve will move to that side to shut it off to maintain brake function in the other side. It will contact the brass plunger on the switch, turning on the brake light. It also happens that corrosion builds up in that area, shorting out the switch, also turning on the light. Finally, the two wires in the connector are actually shorted out by the switch, so that when you pull it, it also triggers the brake light. It's to prevent people from trying to ignore the shuttle valve switch activation by pulling the connector.

In any case, fluid in that area means the proportioning valve is leaking inside, and should be repaired or replaced. Eventually the leak will get bad enough that you are going to start losing braking power.

 

It still seems like a very long time for that thing to be failing and not actually fail. I would have expected it to worsen or fail after such a long time. At least I would expect some drive-ability symptom to make me concerned. So the switch is mechanical, and normally closed. Does that mean the switch was never bad? Guess so. I wonder why when I change it out the problem (leak and light) goes away for a period of time. Anyway, I am guessing no one is going to have a kit for this. I will probably have to head back to the junk yard for a used one. Since the switch was over $60 at the dealer, I am betting the prop valve is hundreds. Kind of wish it would run bad, not have meat cooler a/c, or something so I could just throw it away. Oh well. Going to talk to Garret this morning, maybe he can help me out with some ideas. Already searched online for parts and there are not any, just like the switch. Gonna look at Summit to see if a compatible adjustable unit is available. Just like my old Honda, when they don't make your parts any more you have to make your own.

 

hate to say it, ebay.
want the Ford '60s>'80s with front discs/rear drums proportioning valve.
has the correct front/rear pressure bias. direct replacement. about $40 new with the switch.

Cardone and Dorman are dropping our loved Aeros, a shame.

 

I'm guessing the leak in your valve got slowly worse over time. Can't say as to why switch replacement temporarily fixes it. If you can go back to those same vans in the junkyard where you got the switch, you can now pull one or more of the prop valves, and see if they have any fluid in that switch port.

As I said, the switch is actually just a single contact between the plunger and the shuttle valve. But the wiring going to the switch has two wires that must be connected together, which is done in the switch itself. So a short between the plunger and ground will trigger a light, and an open between the two wires going to the switch will also trigger a light. They're sneaky that way.

I'm not sure if a prop valve designed for a shorter (height) car will work for a van, with its much higher center of gravity. My feeling is that the pressure split would be much more front biased, given the expected weight transfer of a taller vehicle. Either that, or the ABS circuit is going to work overtime. Sometimes I feel like the brakes on my van are already over-biased for the rear, as a sudden stop at low speeds can cause the rear brakes to lock up.

 

I am still wondering what the internals of the valve are like so I will likely pick one...or two up at the junk yard and tear into it. As for the aftermarket par, it is fully adjustable. With yrs of racing experience, I think I will be ok getting it adjusted. Still, If I can make the junk one work properly, I will use the oem part. Are they any special bleed instructions with this system? I have a Haynes manual, not a big help.

@96_4wdr is there a specific search or model I should look for?

Thanks