booster or vac issue?

[vandy]

so here is the deal. start the truck up and the p brake red dash light is on, if the ebrake is set or not. it typically goes off after a few min of idle, or at the worse when i start to drive down the street it goes off in a couple of feet.
it typically stays off. but i have noticed a couple days ago, at a LONG stop light with my foot on the brakes it turned back on. i put it in park took my foot off the brake and it went off. also had my kid in the truck and was at a light. pumped the brake and made the truck "dance" (gets a cheap giggle out of the kiddo) and it came back on.
im guessing the light is a low vac. idiot light?

i think with the truck running and my foot on the brake i think i can hear what sounds to be a vac leak.

so what do you think... vac pump or booster? i think ive got a brake in the weather for a few days so i can drive my car. think im gonna tear into it

[madpogue]

The PRIMARY function of the light is for HYDRAULIC failure. FIRST thing to check is for a failed master cylinder, leaking lines, leaking wheel cylinders, leaking calipers, etc. This is the most critical thing to check, as it's the potentially most dangerous one.

SECOND function is vacuum failure. Even with the vacuum working properly, repeated pumping can overwhelm the vacuum capacity and turn on the light. The pedal should start to get hard when this happens, too. So what you're seeing with this test may not be related to the light coming on at other times.

THIRD function is to indicate you've activated the parking brake. That's a simple switch on the pedal/release assembly. If that switch is acting up, or the wire to it is shorting to ground, it'll turn on the light.

When it goes on, other than the test you did, is the pedal too hard, too soft, or normal? When you have to hold your foot on the brake to hold back the tranny at a stop light, does the pedal slowly drop?

[vandy]

ive had the truck for about 6 months. the pedal seems softer and brakes are less aggressive than when i first picked her up.

ive got no leaking brake lines, wheel cylinders etc.

yeah the pedall gets softer the longer i hold it

[vandy]

Just got back in her running i can push the pedal. It will slowly go to the floor and i get a light about ⅔rds of the way there

[madpogue]

This is sounding hydraulic. Could be an internal failure of the master cylinder. No external leak, but fluid leaks past the master pistons rather than traveling through the lines, and this allows the pedal to fall.

Test - with the engine OFF, parking brake OFF, pump the pedal several times until it gets hard; this expends the reserve vacuum in the booster. Then push hard on the pedal and hold. See if it slowly fades down. If it does, that suggests an internal M/C leak. Then pump the pedal, still with the engine off, and see if that makes it stiffer. That would further support the m/c failure idea. But you have to do this test with the engine OFF, with NO boost, as the boost can confound the test.

It would also be worth pulling the rear drums, just in case there's a slow leak at one of the wheel cylinders that you can't see yet from the outside.

[vandy]

Engine off it will pump stiffer but i can still push it to the floor without much effort.

[madpogue]

Okay, if it goes to the floor engine off, then it's gotta be hydraulic. Vacuum is out of the picture in that case. As said, pull the drums, in case it's a slow leak you haven't seen externally yet. But that's the thing; this is not something to take lightly. A small hydraulic leak/issue can become a major/complete hydraulic failure at the worst time.

[vandy]

Why would it kick my ebrake light on if it were a hyd issue? I found a booster/master combo for 111.00 my price is $61.00. Hopefully this weekend i can take a better look at the rear drums.

Thanks for the insight sir

[creekfreak]

How cold is it where you are? My ebrake light has been coming on in the morning when I fire her up and I've attributed it to it being like minus 20 outside.

[madpogue]

Re-read post #2. It's not the "ebrake light". The light comes on for THREE reasons. The parking brake is the LEAST important of them. The MOST important function of the light is to indicate hydraulic failure; this is how this light has functioned on virtually every vehicle sold new in the US in the last 50 years. TBH, this is something that should have been covered in everyone's Driver's Ed course.