HELP!!! Does Soft pedal + BRAKE light intermittant = Master cylinder ???

[markrinker]

Today I noticed a few hard pulsations back through my brake pedal, usually when shifting from park after idling during stops. Then tonight, started getting bad pedal fade combined with BRAKE light on dash.

I've never done a diesel master cylinder, just gassers that are vaccuum assisted. What gives a diesel power brakes? Is it electric? Should I assume the entire unit, or look for a simple problem? (Fluid reservoirs are full.) Anything else that could be the culprit here?

I have to be back up and operational tomorrow...any troubleshooting advice appreciated!

[rjschoolcraft]

The power assist is provided by hydraulic pressure off of the power steering pump. If you look, the master cylinder is mounted to the "Hydroboost" unit which has steel tubes running to it from the power steering pump. The pulsations could be the hydroboost unit going bad, but I felt something like that when my crank pulley was failing.

The Brake warning light indicates an imbalance from front to rear brakes (one way or the other). Check to make sure you don't have any leaking wheel cylinders or hoses. This could cause a pressure imbalance that would turn on the warning light before the reservoir is depleted.

I'm assuming you're familiar with the pulsations caused by the ABS activation when moving and that what you have described is not that. Although, I suppose a failure in that system could cause it to activate even when stationary. I can envision a scenario where that might give the symptoms you describe (kind of thinking as I type here). Check your codes. The ABS codes read out on a different pin than the normal power train diagnostic codes. You short pins A and H in the ALDL (OBDI). The ABS light will flash (like the SES light).

[markrinker]

Good info. I think the key here is the soft pedal. Same mushy feel with too much travel as my gassers have done in the past, also no ABS light.

I pull codes and check into it. GMPartsDirect.com lists a rebuild kit for the K3500 with hydraulic brakes.

[markrinker]

I got off cheap on this one. Rusted brake line on the rear axle had developed a pinhole leak. It was spraying up, and a little hard to find at first. The slightly low fluid level in the resevoir was the first clue that pointed to a leak.