brake load sense valve

[odee]

have an 86 gmc c30 with the load valve above the rear axle.The linkage was unhooked from the diff when I got it. Would it be okay to bypass the valve completly? thanks

[chengny]

Yes, even GM gave up on this concept - back in 1988. TSB on the issue of overly sensitive rear brakes


GMC NUMBER: 88-T-151
GROUP: 5 - Brakes
DATE: August, 1988
CORPORATE
NUMBER: 865010R
SUBJECT: REAR BRAKE SENSITIVITY - HEIGHT SENSING VALVE REMOVAL
MODELS: 1984-86 C/K 3500 AND 1987-88 R/V 3500
THIS BULLETIN IS A REVISION OF TRUCK SERVICE BULLETIN 85-B-79, DATED MAY, 1985, UPDATING FOR MODEL YEARS 1984-1988.
1984-1986 C/K 3500 and 1987-1988 R/V 3500 series trucks are equipped with a rear suspension height sensing device that optimizes the brake proportioning valve setting for the load the vehicle is carrying. The height sensing system adjusts the brake balance in vehicles subjected to a range of loading conditions. Occasionally, vehicle modifications by body builders or owners influence the height sensing device. If a vehicle has had rear suspension modifications that affect its trim height or the spring rate, and the driver comments about the brake modulation characteristics of the vehicle, it is recommended that the height sensing system be removed and the brake system be revised as described below:



It goes on to show a procedure that involves a lot of expensive (and currently NLA) brackets and fittings. But it is most often done using off the shelf brake lines/hoses/fittings and usually comes out looking like this:

[andrewmp6]

Yeah most unhook them or fully remove it.

[odee]

thanks guys. Will do when I rebuild the rear brakes.

[andrewmp6]

Ford used something like it in the 90s on the taurus so it wasn't just gm doing it lol.

[GonicGM]

IIRC, GM starting using these on 30 series trucks in 1982. My 1982 manufacture 1983 K30 has it. I think it was introduced on 3/4 ton in 84 or 85. My 83 K2500 8600 GVW didn't have it. Mid 80's Jeep Commanche had a similar K-H valve.

I use it myself as it helps with rear wheel lockup on unloaded trucks, especially when carrying a plow up front. Indexing of the valve shaft is critical for optimum brake performance. I actually fabricated a pointer to hang off the "D" profile part of the shaft to indicate the step position relative to the plunger travel limit so I could fine tune it.