^What he said. But I disconnect the return line. Low pressure.
Run engine for a couple seconds line into a bucket, replace lost fluid. Repeat a few times until “old” fluid comes out clean.
Helps to have 2 people for this task.
On newer cars, I do a “lazy mans flush” on PS and brake fluid if it’s still decent fluid. Suck out all the fluid in reservoir with turkey baster or vac pump. Replace, run, repeat several times. Brakes I run the vehicle for weeks between replacements so the fluid “mixes”.
Not as complete as first method, but think this way. mic you get 50% first time, next time will be 75%, next 87.5, next 93 etc. it wastes good fluid but quicker cleaner easier.
Not if fluid is trashed though imo.