Proportioning valve

[Steve83]

I'm not saying that the 80-96 F series doesn't have a hold off valve...but I THINK it is in there.

It doesn't. The fluid control valve is a residual pressure valve, which accomplishes the function of a proportioning valve, but isn't a proportioning valve.

(click this text)


And it's only on the rear port of the MC, which serves only the rear brakes. This is from the '96 service manual:

Dual Hydraulic Brake System

To protect against total brake failure caused by worn or damaged parts, the dual hydraulic brake system is split into front and rear subsystems. The front brakes are served by one brake master cylinder piston and outlet port and the rear brakes are served by the other brake master cylinder piston and outlet port.



Brakes, Front
The front disc caliper used on F-150 and Bronco vehicles is a dual pin sliding disc brake caliper, single piston type. The system has a dual brake master cylinder and front disc brake hub and rotor.

Four-wheel ABS is standard on Bronco. The ABS wheel end includes an ABS sensor mounted to the knuckle and a tone ring pressed onto the rotor. Refer to «Section 06-09A» or «Section 06-09B» for servicing the ABS system.

The front disc caliper used on F-250 and F-350 vehicles is a dual pin sliding disc brake caliper, dual piston type. The system has a dual brake master cylinder and front disc brake hub and rotor.

The front disc caliper used on F-Super Duty series vehicles is a rail slider disc brake caliper, dual piston type. The system uses a dual brake master cylinder and a Hydro-Boost brake boost.


Brakes, Rear
The rear drum brake system includes:
l brake drums (1126)
l brake shoe retracting springs (2035)
l brake shoe hold-down springs (2068)
l brake shoe hold-down spring pins (2069)
l leading and trailing brake shoe and lining (2001)
l rear wheel cylinders (2261)
l LH rear brake backing plate (2212) and RH brake backing plate (2211)

The rear drum brakes are a standard system with rear wheel cylinders mounted at the top of the rear brake backing plates.

The rear brake system is a more complex system. Trucks are designed to carry a wider range of loads under more severe driving conditions than passenger cars. As explained earlier, the heavier the load in the rear, the more braking force can be applied.

It is also true that trucks often run with light or no loads. Whatever the loading, truck rear brakes have to provide sufficient braking force without causing the rear wheels to lock up prematurely.

 

[toddz69]

It doesn't. The fluid control valve is a residual pressure valve, which accomplishes the function of a proportioning valve, but isn't a proportioning valve.

(click this text)


And it's only on the rear port of the MC, which serves only the rear brakes. This is from the '96 service manual:

How does a residual pressure valve accomplish the function of a proportioning valve?

Todd Z.

 

[Steve83]

By holding a tiny bit of pressure from the last pedal application (residual pressure) in the rear brake system so that it's already biased for the next application. But the bias is fixed - it doesn't change for varying vehicle suspension loading, like a proportioning valve, which mechanically changes the bias by comparing the front & rear pressures, as in a common mechanical pressure regulator (like in a shop air system, or on a propane/LPG tank).

 

[Big Slim]

seems like i should put the proportioning valve after the H-block so that both sides see the same pressure for the warning light?

 

[jamesroney]

seems like i should put the proportioning valve after the H-block so that both sides see the same pressure for the warning light?

Yes, if you want to keep the brake warning light from coming on when you apply the brakes. Your logic is correct.