Long Time Lurker Here first time post.
So I have a 51 Chevy Pickup. Set on an S10 Frame. I installed a 2004 Silverado LS1 with 4L60E. Lots of what I was able to accomplish on my own was from hours of reading you alls posts. That was almost 4 years ago. Truck has run flawless. This summer with extended vacation I decided to swap out the stock truck Oil pan (Too Deep) for the Holley shallow Pan. I needed more ground clearance. Had to unbolt motor and raise 3" in order to accomplish this job in the truck but I got it done. I went to drive the truck and brake pedal goes straight to the floor.- Now I have 0 brake pedal. I did not have any brake issues before this oil pan swap. Never had any leaks with brakes what so ever! Did not touch the brakes during this other work.

I assumed it was just a bad master cylinder- So I purchased a new one. Bench bled master cylinder installed. no luck Pedal still goes to floor with truck running.
Purchased a new vacuum booster & installed it- No Luck. Pedal goes to floor with truck running.

I capped off the master cylinder with solid plastic plugs while mounted in Truck (Isolated without brake lines attached) Only then did Have a solid pedal with truck running. So this tells me that the master and booster can hold pressure when not hooked up to the front and rear brake lines.

I do have a GM OEM proportioning valve that came on the S10. I have never touch any adjustments on it. Brakes have always been perfect.

No Brake Fluid Leaks that I can see anywhere under the truck. (4 wheel disk brakes)

I do have a solid pedal with truck not running but when I start engine- pedal quickly goes to floor.

I inspected front brake lines around the motor mounts looking for something that I may have done while lifting motor- don't see any smoking guns.

Front and Rear brakes are disk- never had any brake issues in 4 years until now.

Spent 2 days on this now- I am Stumped!