I can't verify this but there is a good chance that the proportioning valve most bronco vendors are stocking comes from the same manufacturer. We actually just installed one on a Bronco and after bleeding the brakes it started leaking from the front where that rubber plug is. We talked to our rep and he said that one of the ways to create a leak is to use a vacuum assist pump to bleed the brakes. We had in fact used a vacuum pump. He sent us the replacement unit and instead of replacing the whole thing i just un screwed the relief valve and swapped it into the one already installed. Then i gravity bled all four corners and have had zero issues since. I inspected the pieces that came out of the leaky one and I did see a small nick in the rubber seal. I have no clue what caused the leak but I have a few more Broncos that are getting brake jobs soon so i will be keeping a close eye on the p valve to see if using a vaccum assisted pump creates a leak.
Please feel free to prove or disprove their theory just don't beat up the messenger.
A few posts up, I give a theory opinion. I also had to replace the inner valve and seal assembly due to the leaking at the rubber plug. I believe the piston style seal gets cut during the assembly process of the proportioning valve due to the sharp edges of the brass and tight fit of the seal in the bore. The reason it takes a while to leak and that it varies from vehicle to vehicle is the severity level of the cut in the seal and the location of the cut. As the system is pressurized, the cut begins to grow until the leak path is created.
Being in the seal business for the past 15 years, I see it all the time and a large portion of seal failures / eventual leaks happen at seal installation rather than the harshness of the application. Sharp edges (especially with aluminum and brass) are detrimental to rubber seals (and fingers).
In my proportioning valve, I found the sliver of the rubber seal detached from the seal. Unfortunately, these seals on the inner valve are not a simple AS568 or standard metric oring - from memory, the seal on the inner valve piston looked to be a custom molded shape as opposed to a round CS or quad shaped CS. They might be square or rectangular, can't remember exactly but I'm pretty sure you can't buy just the seal off shelf.
I believe all of the vendors are aware of the problem and generally don't challenge the fact that this is a problem area of the proportioning valve. The prop valve manufacturer should take a look at the process or offer a standalone replacement seal only kit IMO
DJs74
