Hey Guys need help. On the way home to night the brakes started acting funny. When I press the peddle down it stops the truck but if I sit there holding the brake it starts pulsating and dropping towards the floor. Here is where it gets real strange occasionally the peddle pushes back and then drops again and pushes back. Keep in mind I am not moving. I am due for some new brake pads. Could this be what's causing it? Some additional information, it is four wheel disk brakes.

 

Are you loosing brake fluid? If not it's probably time for a new master cylinder.

 

I did not see any on the ground but I did not look around the Master cylinder. It is filled about 1/8 to 1/4 from the top.

 

You'd be loosing fluid fast with what your describing. I'd guess the cylinder walls are scored in your master cylinder and its allowing the fluid to get back around the piston.

Try bleeding the lines first (start furthest from the master and work closer) that way you can make sure it's not just air in the lines.

 

Changed the brake pads today. I did not bleed the system. Used a c-clamp to open the calipers. Got every thing on the took for a test drive. Appeared to have fixed the problem, or so I thought. When I got home and parked it, it started doing it again. It only seems to do it when I am sitting still and holding down the brakes. Also it only appears to do it after the truck gets good and warmed up. I do not understand how there could be air in the line because it stops fine and I have never cracked the system open or run low on fluid. It only happens when I am sitting still and it just started out of the blue about two days ago. I do not mind buying a new master cylinder but only if it will fix it. Help.

 

Sounds like the booster getting uneven vacuum to me.