'79 Suburban brake proportioning valve

[jimfulco]

'79 Suburban, half-ton, 2WD, gas engine, 260K miles. My proportioning valve seems to be leaking from some kind of opening underneath a hexagonal fitting on the far right side, which feeds the rear brakes. There was a tiny (~1/4" diameter) star-washer looking thing and a rubber thing in the hole. I can't imagine a tiny rubber doodad holding back hydraulic brake pressure for 30 years, but it just recently started puking fluid all over the place. Any idea what this opening is supposed to be for?

[N2TRUX]

Anyone?

[menace121978]

pics would help buddy... i replaced mine a few years back. was leaking from rear brake lines/proportion valve also. just kept getting worse and worse till i had to fill up the rear resevoir every week.

[jimfulco]

It's leaking from under the black rubber thing on the fitting on the right side of the pic. Nobody in town has the valve, not even the Chevy dealer. I need to know what the purpose of the black thing is so I can decide whether or not I can plug it with something and get the truck back on the road.