Few months back my master cylender went. Pedal would just hit the floor. So i replaced the master and bled the system. Brakes were back. Its been about 4 months mabey and the symptoms are back. Pedal is sinking to the floor when im driving. My master has a very little bit of thick black gunk flating at the very top of the fluid. Is this just another wrecked master cylinder or is it something else thanks guys

 

have you bled the system again? and what year is the truck? if you have drum brakes on the rear then you mite want to check the rear wheel cylinders.

 

i bled the system and have replaced the wheel cylinders recently