Vacuum hiss when brakes applied

[DirtyJim]

My 72 F100 has had this issue for years. The brake light stays on all the time and you can hear a loud hiss when the brakes are applied. The master cylinder was just replaced and the brakes work great. The pedal stays firm throughout braking. I just want to be sure that the truck is safe for the most part. Is this the booster that is leaking? I also heard that there is some sort of switch on the frame that is related to the brake light somehow? Any ideas?

[heep70]

Brake light issue: look under the dash behind the brake pedal arm and you will see a plunger switch. It probably needs to be adjusted. Move it closer into the brake pedal arm to where the plunger is pressed in. Leave a little play.

The hiss: is it constant (hisses the whole time you are on the pedal)? If so the diaphram in the booster is bad. Your idle will also change when you are holding the pedal. Napa sells new boosters. OR does the hiss only occur when you press the pedal down but not while you are holding the brakes? If so, that is the air entering that back of the booster from inside the cab. Not an issue, although some people seem to think it is. To each their own.

[DirtyJim]

If I hold the brake pedal to the floor for 5 minutes, it hisses for 5 minutes. I'll check the plunger before I head out the door to work this evening and looks like I'll browse the net tonight for a new diaphragm for the booster as well vIs the plunger under the dash for the brake lights on the back of the truck? I was referring to the idiot light on the dash that says "Brake" on it...sorry for the confusion.

Thanks for the reply! It will be nice to have the old truck closer to new again!

[68F250]

Are you talking about the little red light on the dash next to the e-brake or the brake lights on the back of the truck?

You can't just buy a diaphragm for the vacuum booster, you have to replace the whole thing with a rebuilt one.

[willowbilly3]

Sounds like you need to reset the eqaulizer on the frame. Best done with 2 people. One person holds slight pressure on the pedal while the other cracks open a line at ther master cylinder. The person on the pedal needs to stop pushing when the light goes out and then you close the line, and then they can let off the pedal. If it doesn't go out on the first line, do the other one. If they are pushig too hard the balance valve will go past center and trip the light again and then you have to do the other line to get it back. It's touchy, expect it to take several tries before you can get the person on the pedal to savvy.

[DirtyJim]

Yes, I was talking about the little red light on the dash.

I realized after I posted that I probably couldnt replace a diaphragm inside the booster....I'll be looking into getting a replacement soon.
Regarding the equalizer on the frame: That sounds about right to me. My father hasa 68 that experienced the same problem. He said it was some kind of proportioning valve on the frame that needed replaced. It probably just needed adjusted. I'll give that a try.
Now, without having an extra person available to push on the brake pedal, is there anything I can do with a vacuum pump to complete this chore? That I do have; it would be great to find another use for that great little tool.

[DirtyJim]

I found the proportioning valve, or equalizer on the frame. Checked the wiring to it and tapped on it with a hammer (you always gotta try that first you know ) a couple times.

If I'm understanding correctly, the procedure is simply to break loose one of the brake lines leaving the master cylinder while pedal pressure is applied in order to lower the system pressure on that side of the valve. Similar to brake bleeding; if no change in light status after a couple of tries, switch to the other brake line.

This sounds like it should do the trick, provided the valve on the frame is still functioning, but it sounds like I'll have to lose brake fluid in the engine bay to achieve this. If this system just tells me that pressure was lost in one or the other brake systems, then couldn't I just bleed the brakes normally with a vacuum pump? There really should be a cleaner and easier way to do this chore rather than needing two people. I replaced the master cylinder only a few months ago and bled the brakes by pulling a vacuum at each wheel. The light still stays on even after that procedure, so maybe the valve is bad....lets hope the hammer trick works.

[68F250]

Been thinking about this since you posted and I don't think the vacuum pump is gonna work. The pressure differential valve need pressure on one of the inputs to push that valve one way or the other. Drawing a vacuum on one of the outputs isn't strong enough I think. Sometimes that valve gets stuck and you have to manually recenter it.

It doesn't take much pressure to center it. Crack one of the lines out of the MC and wrap a towel around it, you won't loose that much fluid. If you don't get to it right away, just unplug the connector from the valve. The warning light really gets hot and will melt the red lens.

[DirtyJim]

Ohhh...good point about the input. I'll do the old towel trick. Now I just need to rent a second person.