It sounds like your master cylinder is bypassing internally. Over time, the cup seal in the master cylinder can wear out and cause the fluid to bypass the seal, which results in a sinking pedal. Think of it along the same lines as when piston rings wear out... you burn oil where the oil gets past the rings.


There are only a handful of things that cause a sinking pedal while stationary. One is a leak. Then a master cylinder, then a brake booster. The booster can go bad and over assist you in braking, causing the pedal to sink, but that does not affect the actual braking ability of the truck, just makes for a squishy pedal.


My money's on master cylinder.

 

Ok, your descriptions make sense. But this is the third vehicle (had a 94 and 95 Intrepids) I've had this same occurance with...where I push the piston back with a clamp to make room for new pads; and RIGHT after I have this squishy pedal issue...am I maybe blowing the seals in the master cylinder by pushing the fluid backwards up into it?

 

More often than not, master cylinder issues do start to occur after brake service.

 

Thanks for the replies. Is there a way to pressure test the master cylinder before I go buy one? Also, the reservoir rocks back and forth on the cylinder, is this normal, or likely air seeping in from that point? I saw a little bubbly going on in the black washers as I rocked it, but I had JUST sprayed brake cleaner all over the area between the cylinder and reservoir to make sure it is nice and clean so I can see if any fluid leaks over the next couple days.

 

No offense, but you are having some issues, that maybe only a reputable brake shop can figure out, and since brakes are a rather important feature , my suggestion would be to contact one of these shops before these brakes really get you in trouble. I know that you are one of the people on here, who want to do as much as possible on their rides, and I am also one of those people, but when I have a problem such as this, I always remember what Dirty Harry said in Magnum Force, A man's got to know his limitations. Good Luck

 

Yes they make pressure gauges for master cylinders, don't think they make cheap ones. My matco set was 300 I think. So for the cost of the gauge you could take it somewhere for a brake inspection and have it confirmed and maybe fixed...

 

I have finally figured out the problem with this, I don't remember what I did some time ago, that made it work a lot better than when I was saying was hardly driveable...but what I did this time was pull the rear calipers off and sanded down the slide pins to clean them up and make sure there were no burrs, and wire wheeled the bore that the pins so through, and lubed the snot out of the boots, the bore and the pins. While I always make sure to lube those pins and make sure the caliper slides back and forth, apparently, it didn't slide enough, and it would take a couple pumps to get the caliper to slide and squeeze, and I was having to push the pedal harder since I was lacking rear braking power. It is A LOT better, but not perfect, so I'm thinking the front calipers may be hung up a little, or the pads are worn/glazed from doing just about all of the stopping, tomorrow I will be pulling the fronts all apart and making sure everything is all clean and relubed and sand down to make sure no glazing is present. Hope this will help someone in the future