brake sinks

my brake pedel sinks...i have 96 sub. i just changes a lot of things on the brake system. front pad and rotors, rear shoe and drums, i also change the master cylinder. bleed the whole system but still the same. can anyone help me. been trying to fix this shit for the past two week

Are you losing fluid?

Did you bleed system correctly?

Sometimes this can be a pain in the ass.

Best to pressure bleed...

did you bench bleed the master before you installed it on the truck?? If not then there is a ton of air in the system and your best bet would be to power bleed it. Also any leaks in the system anywhere. Like the rear wheel cylinders??

If you have tried all the proper bleeding methods (starting at furthest wheel from master cylinder and working your way closer). Does your vacuum booster make any noise like air moving or letting air out of a tire when you apply the brakes? Then it might be the diaphram in the booster is leaking. I had a bad booster on my 97 Burb and the pedal would almost be at the floor before it began to stop. A new vacuum booster is about $150 and IT IS a pain to install.
I actually had it to the dealer 5 or 6 times during my extended warranty period to complain about the noise(air) and poor pedal feel and they told me it was "normal". However, about 20k miles later the pedal started going to the floor and it felt hit or miss if it was going to stop or not. Also check your vacuum at the manifold before you invest in a booster, I think the spec is 14 pounds of vacuum.

good point !!!

yes i bleed the master cylinder first B4 i install it. i did everything by the book on the bleeding system.

If you decide to get a vacuum booster remember to check the vacuum first. And I paid $156.90 plus a $50.00 core charge at a Federated Auto Parts store and it came with a lifetime warranty. After a weekend of trying to install it by laying backwards in the seat to removing half the dash, I took it my local service station that has a good reputation with American cars and it was done in less than two hours for $100.00 out the door! The brakes work great now but I still have to adjust the rear shoes "by hand" to keep the pedal feel at the top.