Hey Guys,
I've developed a problem with my brakes almost overnight and it is one of these mysteries I can't seem to understand...

I've checked for the usual/obvious causes with the exception of pulling off a rear wheel and checking the rear brake cylinders (yet).

Figured I'd poke around here first...as my suspicions lead me to believe it's something else. I can never seem to get away with something as easy as 'oh it's just low fluid'.

Master cylinder replaced 2 years ago (new/rebuild parts store unit)
Brake booster replaced 2 years ago (same time w/ new)
new fluid at the time of change
brakes properly bled and had been working fine
fluid is currently topped off

When I depress the brakes to go to stop, the truck doesn't want to stop unless I put excessive pressure on the pedal, as in like using 2 feet to get enough force to stop. The pedal kind of feels as though it's going to the floor as the truck slowly comes to a stop.

Then, this morning when I went for a drive the brake light did come on as I was depressing the pedal to stop. I would accelerate and depress the brakes again just to slow down a bit and the light went out.

Seeing as how I don't see any obvious leaks, a loss of fluid or anything out of the norm I have to assume it's an internal issue...SO where do I begin to look and how can I test things without just throwing money at it? I know the booster is holding vacuum so I've kind of ruled that out as well.

My thought was maybe the proportioning valve is not working properly? Or could the new master cylinder is shot already? It seems as though something isn't getting enough pressure possibly causing the proportioning to do it's thing OR the proportioning valve isn't holding in turn clicking the brake light on.

Has anyone experienced a similar problem after already having "new" brake parts put in?

Guess I'll have to fire up the ole' powerstroke and take her to work today! oh dang... guess it could be worse. ha-ha

Thanks so much for any input....

 

probably a wheel cylinder ~Bill

 

How is the engine running? Ok? Check the vacuum line going from the booster to the intake manifold.

 

Engine runs ok. there's a slight misfire I haven't dedicated the time to tracking it down yet but otherwise its been running ok for a tired motor with 167k. I did put a vacuum gauge on it and I'm pulling around 19" of vacuum give or take. Needle hold relatively steady so I would say it's safe to assume there aren't any MAJOR engine issues (sticky valves/lifters, etc...)

I wouldn't be surprised if the so called new master cylinder I put in was bad. I've only seen one brand new part go bad out of the box and now I have to remember to always keep that in the back of my mind...it's possible.

edit: vac line to booster is good too. It was replaced roughly two years ago. When I pulled the line from the booster (the engine had been off for several hours) it was holding a fair amount of vacuum or so I think

 

Sounds like the master cylinder is failing.

Without the vacuum line attached to the booster, is it even harder to press the pedal down? If it's the same, could be the booster failing instead.