Beezer

ok sorry if this has been discussed a lot before but i couldnt find anything with the search feature.

so every once in a while when the i start up the truck and take off without warming it up i notice that the brake pedal goes almost all the way down feeling kind of spongy and it feels weak at stopping the car but when its warmed up it stops normally. the pedal doesnt feel like its pushing back or anything just really easy to push all the way down. could it be a vacuum leak or would i have to change out the MC? it doesnt leak or anything but the fluid does look really dark im not sure how old it is because i had just recently bought it. any ideas? thanks.

desertcamper67

First off and cheapest thing to start with is to bleed out all that old brake fluid. What kind of truck is it? If it is an older model you may not want to let the pedal go all the way to the floor when bleeding. On an older master cylinder this could damage it. If you're not sure how low to let it go down, just put your other foot under the pedal so it only goes part of the way down until you have clean fluid coming out of each bleeder.

It doesn't sound like an old fluid problem but clean fluid has a higher boiling point and usually gives a better pedal feel.
You may indeed have a bad master cylinder right now but you do need to remove all that old dirty fluid to make sure.

milehigheric

If it goes away I can't see it being air in the lines...air in the lines is air in the lines it doesn't go away. Perhaps the oil was replaced with the wrong type...I haven't had much experience with braking systems (never needed to) but I would assume there would be different ideal operating temperatures for different class oil. Maybe until the oil is warmed up the brakes feel spongy? Not to expensive to empty/flush/replace the oil anyway...maybe worth a try.

Beezer

ok thanks for the replies well, this is for a 86 pickup 4x4. the morning after i posted this my brakes locked up after a stop sign and would gradually loosen up after a while so i did this all the way home when i got home i pulled the MC and noticed the piston (i think thats what its called) wasnt retracting all the way back. I remembered seeing something it here saying that the v6 4runners booster and MC was a direct fit so i pulled one off the 4runner i had laying around and now it feels and stops great although it could of been my booster too cause at first i just changed the MC and nothing happened the brakes werent biting hard at all so i had to remove and pull the boosters.

Beezer

oh one more thing I tried bleeding the rear drum brakes but nothing ever came out of the bleeders is it normal or do i need to replace them too? But i did bleed what looks to be the proportioning valve up under the bed by the gas tank.