97 Chevy k1500 Silverado brake issue

[Eagle]

Friend has a truck with about 200k on the clock and recently started having brake issues. The pedal is very VERY firm when you first hit it, then slowly softens over a few seconds and the brakes actually then start slowing the vehicle. There is a noticeable hiss that can be heard with all the accessories and radio turned down.

Feels to me like a booster issue... but not certain. Going to take a look tomorrow.

Any thoughts o thee great TCG?

Click to read more...

 

[bikrboy128]

my first guess would be a pinhole leak in a brake line. im sure they're all rusted to shit. i've done tons of brake lines on chevy trucks, but never a booster. but i guess anything is possible with that kind of mileage

 

[Turbocharged400sbc]

ive had the brake booster hoses collapse internally and prevent/block vacum assist

so the brakes dont start to slow you down till after it softens/gets power assist?

or is it that you hold the brakes then the pedal slowly sinks then the truck will move as pressure drops...thats a sign of internally bypassing master cylinder

 

[Eagle]

Once the pedal starts to move, it begins stopping as it should.

 

[SRT41320]

if the pedal will not hold... its a fluid leak IMO

1) rear slave cylinder
2) brake line (has 200k - something has prolly rusted)
3) internal leak at the master (doubtful as you say you hear a noise)

 

[HILROD]

If it's hissing in the cab, it's the booster. Pull and cap the vacuum line to the booster. That will tell you if the noise is gone and pedal should be hard but steady.

 

[Eagle]

Brakes WORK to stop the truck eventually, just the pedal is really stiff and hard to push. The hissing is there too, under light pressure.

 

[HILROD]

.

If it's hissing in the cab, it's the booster.

 

[Turbocharged400sbc]

a hiss durring application is sometimes normal

a hiss while at the up position is usually the diaphram but with modern dual diaphram sometimes all youll see is half the vacum assist disappear...which may be the case

 

[HILROD]

Or if it hisses past a little bit while applied, it's bad.

 

[Eagle]

So to explain the hiss as I've heard it so far, is

*It doesn't seem to hiss at rest at the top
*It hisses as soon as it's touched until depressed to the point where the truck starts to stop, then stops hissing.

 

[Turbocharged400sbc]

yup sounds like the booster is leaking internally

 

[SRT41320]

better question... did this just start happening or was any service done recently?

 

[Eagle]

Just started happening on its own, got progressively worse over the last 3 weeks.

 

[SRT41320]

sounds like a booster - cap the vac source and see if the noise goes away

 

[Eagle]

Booster replaced... truck stops perfect again!

Nice job, TCG!