73 K-20 Experiencing Rock Hard Pedal

[Eddie_H]

Recently bought a 73 K-20. Yesterday the brake light on the dash came on(& has stayed on) & the brake pedal is rock hard, which makes stopping quickly difficult!
The parking break was disconnected when I bought it & still is, so isn't causing the light. What does the light signify? Is it just a general alert that something in the brake system is whacky?
The truck has no brake fluid leaks, the master is full & the system has been bled. Vacuum line from the booster to the manifold is new.
This is my first vehicle, so I'm learning as I go.
My dad had a '72 K-20, but never experienced any brake problems. He sold it a long time ago & so hasn't worked on an "Old Truck" in a long time.
Any ideas where to start?
Master cylinder, vacuum booster, ???
Thanks
EH

 

[chengny]

The brake warning light can come from 2 places. The switch on the parking brake arm and the brake proportioning/combination valve.

The e-brake is not in use and the wires to the switch have been pulled - right? So that can be eliminated.

That leaves the proportioning/combination valve.

Let me say first; I am not familiar with pre-81 C/K trucks and so I will be speaking in general terms.

The proportioning valve - cross-sectional view shown here:

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and located here:

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The P/C valve has a spool that shuttles left and right. If it moves too far in either direction, it trips the switch and you get dash indication of a brake problem.

Just as a test pull the single lead that connects to it and confirm that the dash light goes out.


Description of operation:

These vehicles use either a two function or three function 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.

Your situation is unusual because the spool generally only shifts far enough to trip the switch when there is a major loss of pressure in either one of the hydraulic circuits. You say that there is no evidence of fluid loss and the MC is full.

If removing the lead to the P/C valve extinguishes the light, the spool will have to be re-centered. Resetting the spool to the middle to middle position and opening the switch (extinguishing the dash light) is kind of an involved procedure.

Try pulling the lead off the P/C valve first and then come back.

 

[Eddie_H]

Re: Hard Pedal & P/C valve

I'll check what you said & post a reply. Would/could this account for the pedal being so hard & braking so difficult?
Just curious.
Thanks

 

[chengny]

Good question. IDK - maybe you are only onthe rear brakes?

We'll find out why - I'm sure.

BTW - is the degree of brake pedal travel normal?

 

[Eddie_H]

Thanks. I'll post what I find.

 

[89Suburban]

Thanks. I'll post what I find.

Any luck?

 

[Eddie_H]

Re: Rock Hard Brakes

It's been rainy & cold here, so finally got to unplugging the PV valve on Saturday. When unplugged the brake light on the dash Does go out & comes back on when the lead is re-attached to the valve.
Also brake pedal travel seems to be normal.
Eddie