1983 F250 with 351 and 3spd automatic.

1) When brakes are not pressed, there is a hissing sound coming from the engine compartment. Press the brakes and the hiss goes away.

2) Press the brakes and dash lights inside the cab come on. Un-press and they go off.

Thoughts?

80s

 

The hissing problem sounds like it may be a bad power brake booster.

The problem with the lights is weird. Did this just recently start? Has someone done some custom wiring, like installing a trailer light connector? Just out of curiosity, when you step on the brakes do the parking lights come on (side markers and front ambers)? You may have a short between the brake light circuit and the parking light circuit.

 

I need you to clarify something...

Is it the brake warning lamp that is coming on in the instrument cluster? If so it's warning you of a brake failure of some kind, and you should not drive the truck untill its corrected. Most likely it's coming on due to the brake booster, IE: hissing noise etc...

If it's other lights besides the Brake warning lamp, then please clarify. Thanks.

 

No not warning lights. What I meant was the backlight for the entire dash ... like when you're driving at night.

 

The wire that feeds that circuit (dimmable dash lights, also includes center console) is light
blue/red stripe. You might crawl under your dash by the brake switch and start looking
around for mis-connected wires, another possibility is melted wiring near the light switch.
Something is very wrong somewhere...

 

MAy be a bad ground in the tail light ccausing it to backfeed the park lamp circuit. Take the rear brake light bulbs out and see if it still does it. Also look at the filaments in the bulb, sometimes they break and short to the other filament. The hissing is probably unrelated and if the noise is coming from the booster it needs to be replaced

 

I agree with bashby. Start at the rear of the truck. A crossed fillament in a bulb or crossed wire, or bad ground can back feed through the parking lamp circut, back to the headlamp switch, where the dash illumination and parking lamp wires are connected together inside the switch.

Unplug the frame rail harness from the rear harness to isolate the problem. The connector will be right in the back on the left side frame rail, just around where the bumper brackets bolt to the frame.

If unplugging the connector causes the dash illumination lights to stop working with the brake lights...

Check the grounds and the lamp sockets themselves at the taillamps. Also check for defective bulbs, or any added trailer wiring etc...

If unplugging the connector doesn't change the dash light/brake light problem, unplug the frame rail harness in the engine compartment just below the steering shaft and brake booster. If the problem with the dash goes away this time, then the problem is in the frame rail harness.

If the problems continues as before, then taking apart the dash is required, and removing the instrument cluster to do tests at the cluster connector and various other connectors.

Process of elimination... etc...

 

Okay I've had a chance to fiddle a bit with the hissing issue.

I unplugged the vacuum hose at the power brake booster. I plugged the end of the hose with something, then I started the engine and there's no hissing. I sprayed starting fluid underneath the carb and there's no change in engine level.

I reconnected the hose and I can hear the hissing when the brake pedal is up, but cannot hear it when it's pressed to the floor.

It seems to me the vacuum lines are fine. Bad brake booster?

 

Sounds like a bad booster to me. If you were to spray a little starting fluid around the top of the brake pedal with the engine running, I bet the idle would be affected, confirming the diaphram in the booster is ruptured.

 

Okay I took some starting fluid and sprayed it around the back of the brake booster and also around where the vacuum hose connects to the booster. No change in idle.

But it's idling pretty high to begin with, so maybe that wasn't a good test.

 

you would need to spray under the dash where the pushrod goes into the booster. I'd just replace the booster before it gets worse and you lose power assist

 

Correct. The hissing sound heard inside the truck, by the brake pedal, is 99.99999% of the time from a bad booster. Occasionally, it's from a vacuum line for the cruise that has come loose from the brakelight switch. Though that usually isn't loud enough to be heard.

 

Good call there R.W., I forgot about the vac CC switch on the older vehicles

 

I replaced the brake booster with a brand new one, and the hissing noise is gone.

Now I need to replace the driver-side front rotor. I called the junkyard and they asked me if it's got 9/16 or 1/2 studs. They also wanted to know the front axle weight because apparently there are two possible rotors for this truck.

How can I tell which rotor I need to get?

thanks,

 

Best way is to take your's with you when you go to the wrecking yard.

The stud size is easy, just pull a lug nut and measure the outside diameter of the threaded stud.