When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Ok then what is the point of the sliding valve in the proportioning valve?
I think you are only partialy correct. Yes I understand that there is a split master cylinder. Older cars and trucks used to run without a prop valve. The prop valve lessens the pressure to the rear brakes under normal conditions and is also a failsafe switch. When there is a pressure differential beyond a certain point it flips the valve and turns on the brake warning light.
No, I'm 100% correct. Below is the diagram of the drum/drum brake light warning switch that would be on the OP's truck. It's a warning switch only. The sliding valve in the disc/drum system is also a warning switch only.
I have 405 hours of NATEF certified training under my belt, concentrating in brakes and electrical. I have a pretty good idea of what I'm talking about.
No, I'm 100% correct. Below is the diagram of the drum/drum brake light warning switch that would be on the OP's truck. It's a warning switch only. The sliding valve in the disc/drum system is also a warning switch only.
Same warning lamp switch (C8AZ-2B264-A) used with drum or disc brakes on all 1968/69 Passenger Cars, 1968/79 F100/350 & Bronco, 1969/79 Econoline.
The warning lamp switch used on everything in 1967 (C7AZ-2B264-A), is 1967 only.
Why 1969 Econoline and not 1968? Because, there is no such thing as a 1968 Econoline. While introduced in February 1968, FoMoCo referred to them as 1969's from the get go.
Seems like the difference of applied pressure before disc/drum system was compensated for by different size shoe/drum for front/rear. Cars & trucks both.
That picture is not a proportioning valve though, so what you are saying is 100% correct about it, our trucks have a proportioning valve built in to those, the whole thing is called a combination valve, the proportioning valve is what delays the rear brakes from being applied