[Keith Seymore]

Originally Posted by Formulabruce

I ran into some big issue with GM truck engineering with a weight/fluid Limiter on the rear of my 1 ton truck back in 82. I was shocked how many different weight and F/R ratios they had calculated for their Combo valves, and limiters.

Here's the brake chart for 1982, for full size truck:







The brake RPOs are driven by 1/2, 3/4 and one ton models, in two and four wheel drive, with various wheelbases and GVW ratings, and manual vs vacuum boost vs hydraulic boost.

And here are the characteristics that are controlled:



I would posit the main driver for all the variation is/was FMVSS testing. We had to pass stopping distance tests, at curb and GVW, or we could not sell a truck.

Some were so marginal that we would ask for a specific technician to run the test matrix (ie "I want Wayne to drive this one"). We would shotgun it with "development" tests until we got a pass, and would then label that run as "validation".

Originally Posted by Formulabruce

Its amazing that it has come down to basically 1 Combo valve that's marketed for all.

I'm not surprised the aftermarket tries to sell one part to cover all but that anyone in the industry would sincerely think that would work.

To me it's like selling one carburetor (and associated jetting and settings) and expecting it to cover all engine combinations.

K