The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

chengny · 10-13-2014, 12:21 AM

This is called a "height sensing valve" and is part of the load compensating system. It works together with the proprtioning/compensating valve as follows:

These vehicles use either a two function or three function combination valve. Some vehicles use a height sensitive proportioning valve in addition to the combination valve.
The two function combination valve consists of a metering valve and a brake failure warning switch. The hydraulic brake lines are routed through this valve to the wheel cylinders or calipers. The metering portion of this valve assists in providing balanced front to rear braking by delaying full hydraulic fluid pressure to the front disc brakes until the rear drum brakes overcome return spring tension and the linings contact the drums. The brake failure warning switch portion of the valve activates the brake warning lamp when there is a loss of pressure in either the front or the rear braking system.
The three function combination valve consists of a metering valve, a brake failure warning switch, and a proportioning valve. The metering valve and brake failure warning switch operate the same as those in the two function combination valve previously described.
The proportioning section of the valve proportions outlet pressure to the rear brakes after a certain rear input pressure has been reached, preventing rear wheel lock-up.
The height sensitive proportioning valve provides ideal brake balance according to weight at rear axle. This valve is mounted on the frame and responds to changes in vehicle trim height in relation to rear axle load. Mechanical linkage connects the valve to a bracket attached to the rear axle.

It was a bad idea. GM gave up on it within a couple of years. This TSB was issued in 1988:

Brakes - Rear Brake Sensitivity

GMC NUMBER: 88-T-151

GROUP: 5 - Brakes

DATE: August, 1988

CORPORATENUMBER: 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:

10-13-2014, 12:21 AM	#48
chengny Registered User Join Date: May 2007 Location: New Hampshire Posts: 349	Re: 86 GMC Sierra Classic 2500 questions This is called a "height sensing valve" and is part of the load compensating system. It works together with the proprtioning/compensating valve as follows: These vehicles use either a two function or three function combination valve. Some vehicles use a height sensitive proportioning valve in addition to the combination valve. The two function combination valve consists of a metering valve and a brake failure warning switch. The hydraulic brake lines are routed through this valve to the wheel cylinders or calipers. The metering portion of this valve assists in providing balanced front to rear braking by delaying full hydraulic fluid pressure to the front disc brakes until the rear drum brakes overcome return spring tension and the linings contact the drums. The brake failure warning switch portion of the valve activates the brake warning lamp when there is a loss of pressure in either the front or the rear braking system. The three function combination valve consists of a metering valve, a brake failure warning switch, and a proportioning valve. The metering valve and brake failure warning switch operate the same as those in the two function combination valve previously described. The proportioning section of the valve proportions outlet pressure to the rear brakes after a certain rear input pressure has been reached, preventing rear wheel lock-up. The height sensitive proportioning valve provides ideal brake balance according to weight at rear axle. This valve is mounted on the frame and responds to changes in vehicle trim height in relation to rear axle load. Mechanical linkage connects the valve to a bracket attached to the rear axle. It was a bad idea. GM gave up on it within a couple of years. This TSB was issued in 1988: Brakes - Rear Brake Sensitivity GMC NUMBER: 88-T-151 GROUP: 5 - Brakes DATE: August, 1988 CORPORATENUMBER: 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: