It's a vacuum booster.Forgot to say that my rpms move when i press the brake. Searched it up and a bad booster can cause that too.
Excess brake pedal travel causes excess vacuum loss from the booster--the pssssst sound you're hearing. Excess vacuum loss from the booster acts like a vacuum leak, affecting engine operation.
FIX THE EXCESS PEDAL TRAVEL, see if the "booster problems" go away on their own. It's a total waste of time, effort, and enthusiasm to hunt down booster problems when there's excess pedal travel.
Bleed the brakes with a scan tool, which may/may not also involve bench-bleeding the master cylinder. The master may be faulty, or there may just be a bunch of air in the system including in the existing master cylinder.
You haven't answered any of my other questions, so I don't suppose you'll answer this one: When you replaced the wheel cylinders, did the master cylinder fluid level drop so that air got into the master cylinder from the reservoir?Did you bleed the system with a scan tool? If not, you didn't do a thorough job.
Why suspect the booster when the problem is a low pedal?
Does the truck stop? What is the history of this? Low brake fluid level? Why were the rear cylinders changed--did they leak? Front pads worn-out? How many miles since the previous brake work?