Brake Question

[PJsthnam]

Was wondering if there is any proportioning adjustments. My back brakes lock up pretty easily. It's a 72 3/4 ton with power brakes.

[Keith Seymore]

I'm a 73-87 guy, so I'm a little out of my element, but I'll take a shot at this one:

There aren't any proportioning adjustments (like an adjustment screw or anything) with a production style prop valve. They are simply a valve with a "knee" in the curve so that once you cross that threshold you get a different apply rate of pressure.

Having said that, you could (a) try a different prop valve from a different truck (which would be sort of hit-and-miss) or (b) purchase an adjustable prop valve from one of the race car supply houses (Jegs, Summit, Moroso, GMPerformance Parts, etc).

You could then install the adjustable prop valve in the rear line and crank the rate down until you got the behavior you are looking for.

Also - some later trucks had an additional rear load sensing prop valve, which was a valve located above the rear axle on a "Rube Goldberg" sort of linkage device. When the truck was empty (and the tail end high) the valve limited pressure to the rear brakes; when the truck was loaded (and the tail end down) then it tripped the valve and allowed full pressure to the rear. You could locate and play around with something like that, too, especially if you are going to actually be hauling stuff.

HTH -

K

[N2TRUX]

Thanks for dropping in Keith. Always glad to have another professional with some advice to offer.

[powerdriver 1958]

Is it both back brakes or just one?

[PJsthnam]

Both. It's ok when I put gradual pressure on the pedal, But if I have to hit 'm fast (and I had to once) the tires just skid. When I bought the truck a couple of months ago I was told the brakes were new and the mechanic had said to be easy on the brakes first 500 miles, and that's whats written on the receipt. But its way past the 500 miles. I have driven up and down the street using diff. pressures on the pedal and in my opinion they lock up too easy. Kinda scares me when I'm driving her in the rain.