This has vacuum booster which is matched to the same engine type (5.3) and model years.
So, I managed to get some brake!
So, the problem (confusion) that I have been faced with is: there is no fluid in the lines!! I changed the junyard master cylinder with brand new (not refurbished) ACDelco with no luck and the same problem: hard brake pedal when the engine is off and soft pedal to the floor when the engine is runing.
When I was taking the old master cylinder off, I noticed that when I discounted the brake lines from the MC, there was no brake fluid leaking/dripping out of the MC, eventhough intially I was getting brake fluid at the calipers when I was vacuum bleeding. I didn't think much of it at that time. I thought I got lucky and it is not making a mess! Besides, the level of the brake fluid was not very high in MC.
Fastworward, bench bled the new MC, installed and connected the lines but the same problem. The diufference was that this time around, I was not getting any fluid at the calipers when I vacuum bled them!!. I thought I had a clog some where therefore I took one connection of brake lines apart at a time apart and moved upward. There was no brake fluid leaking/drippg out of the brand new MC?!! I bled the MC again while on the truck, when I push the pedal I get nice flow of the fluid but otherwise the orifices are sealed thight with no brake fluid is coming out of them! So, I connected all the lines back, got a body of mine to press the pedal and keep it pressed to keep the MC vlaves open, and I vacuum bled at the calipers. Now I have a brake although it is still very soft. I believe I still have some air in the lines.
So far the lesson learned: this type of MC does not allow the fluid passage if the pedal is not pressed which means it won't allow traditional vacuum bleed!
What do you guys think?
Sorry about the long post
