Brake Probs

[BadTy]

Hi guys

Have a problem on a customers ty.

If you hit the brakes the truck brakes perfectly but when standing on the pedal he he gets softer and softer an till finally the truck starts moving forward (braking power his completly gone). When the pedal is completly down to the floor the brake light comes on. As soon as you relaese the pedal brake power is retored (no pumpimg requiered).

I have aleady replaced the following parts:

- calipers
- brake tubes
- master cylinder
- wheel cylinders

No fluid leaking.

Somone can help??

 

[dgoodhue]

Re: Brake Probs

ABS problem, I initially would have guess master cylinder problem but I see oyu replaced that. A brake boost leak cause the brake pedal to get hard. Their isn't much more than that to brake system.

 

[MikeRenz]

Re: Brake Probs

isn't the only thing that can cause the brake light to come on the sensor in the proportioning valve?

 

[dgoodhue]

Re: Brake Probs

The brake light come on with a bad ABS as well. I am not quite sure how a bad proportioning valve it going allow the pedal to go soft.

 

[QWERTYphoon]

Re: Brake Probs

Sounds like the brake booster. Unless the rear brake cylinders are leaking (you would see fluid). I had a similar problem and I upgraded to a hydro boost. Make sure the check valve between the brake booster and the intake is functioning.

 

[BadTy]

Re: Brake Probs

The brake light come on with a bad ABS as well. I am not quite sure how a bad proportioning valve it going allow the pedal to go soft.

Can you tell me if the "proportioning valve" is the valve between master cylinder and ABS unit?

The pedal isnt soft. It feels hard. But when standing on the pedal he he gets softer and softer.

 

[BadTy]

Re: Brake Probs

Sounds like the brake booster. Unless the rear brake cylinders are leaking (you would see fluid). I had a similar problem and I upgraded to a hydro boost. Make sure the check valve between the brake booster and the intake is functioning.

Rear brake cylinders are new and no leaking. I replaced already the valve between intake and brake booster.

I saw now the front wheel are stuck. After holding the pedal a few of seconds the rear brake are loose

 

[JuStBlAzIN]

Re: Brake Probs

Can you tell me if the "proportioning valve" is the valve between master cylinder and ABS unit?

The pedal isnt soft. It feels hard. But when standing on the pedal he he gets softer and softer.

Since the brakes are getting hard, it sounds like the booster is not holding up.

 

[BadTy]

Re: Brake Probs

Since the brakes are getting hard, it sounds like the booster is not holding up.

I mean "hard" like should be. I saw now the front wheel are stuck. After holding the pedal a few of seconds the rear brake are loose. Is it possible that fluid on holding pedal comes back to master cylinder? How it can be?

 

[sly dvl]

Re: Brake Probs

I mean "hard" like should be. I saw now the front wheel are stuck. After holding the pedal a few of seconds the rear brake are loose. Is it possible that fluid on holding pedal comes back to master cylinder? How it can be?

Sounds to me like you've got a defective Master Cyl. Wouldn't be the first time I've seen that.
Was it a rebuild or brand new? From a reputable company?
Disconnect the lines from the master, plug the ports and see if you get the same symptoms.
It's definitely not the booster, symptoms don't fit.
Also, most ABS systems don't have a proportioning valve since any proportioning necessary is done through the activation of the ABS. SyTy's could be different though so don't quote me on that...

 

[Black Knight]

Re: Brake Probs

Sounds to me like you've got a defective Master Cyl. Wouldn't be the first time I've seen that.
Was it a rebuild or brand new? From a reputable company?
Disconnect the lines from the master, plug the ports and see if you get the same symptoms.
It's definitely not the booster, symptoms don't fit.

x2 sounds like a bad master, (even though it's been replaced) if you take that out of the equation I would have said a bad master.

Definitely doesn't sound like a booster. :2cents:

 

[BadTy]

Re: Brake Probs

Master cylinder is brand new. But I will replace it today 2. time. Thanks guys.

 

[Vertigo]

Re: Brake Probs

Ironic. I was researching for a diagnosis on a similar problem on my wife's truck. It seems to brake fine except on very rare occasions the pedal will travel to the floor. The truck still has ABS but it seems to be dead. What I found was when the lo/hi pressure accumulator valves fail they allow excess fluid to pass, thus causing the reservoir to drain and the pedal to fall. On her truck you can see the Hi Pressure Acc pushes a little tit out (the one closest to the engine). You can reset it but eventually it'll pop back out again. The service manual states you can try to bleed it out as it's usually caused by foreign material sticking the valve. Who knows.

May be the same issue here if another Master doesn't cure it.
???

 

[Sy#2373]

Re: Brake Probs

Ironic. I was researching for a diagnosis on a similar problem on my wife's truck. It seems to brake fine except on very rare occasions the pedal will travel to the floor. The truck still has ABS but it seems to be dead. What I found was when the lo/hi pressure accumulator valves fail they allow excess fluid to pass, thus causing the reservoir to drain and the pedal to fall. On her truck you can see the Hi Pressure Acc pushes a little tit out (the one closest to the engine). You can reset it but eventually it'll pop back out again. The service manual states you can try to bleed it out as it's usually caused by foreign material sticking the valve. Who knows.

May be the same issue here if another Master doesn't cure it.
???

Crack the lines on top of the ABS unit and let them bleed off some fluid. make sure you use a line wrench on them as that's the only way you can get them loose with out destroying them. I went through 2 brand new GM master cylinders in less than a year before i decided to bleed the ABS. My ABS now works fine and has not given me any trouble in long time.

 

[Vertigo]

Re: Brake Probs

^^Awesome, thanks. Today I found the "H" wire cut on the 10-pin connector. After reconnecting it and resetting the ABS the BRAKE light is illuminated (and I'd suspect the ABS light as well if it was working).

 

[BadTy]

Re: Brake Probs

I replaced master cylinder. Now the brakes are like new

Thanks guys!!!!

 

[Black Knight]

Re: Brake Probs

Good to hear! See kids even brand new parts come out of the box bad, more often then you would think

 

[jpalmer]

Re: Brake Probs

interested in seeing what happens i have a 90 gmc p/u and i went through 3 master cylinders before i had one work, made me scratch my head for two weeks(Dont buy cheap m.s. buy the good ones and have no prob).