Can anyone tell me what this noise is Master cylinder

[LiLFreak]

YouTube

I replaced the whole master cylinder but this one never builds up to full pressure and when you turn the key off it sounds like an air leak maybe internal? I can’t see or feel it anywhere. The seller said it’s doing that because there is air in the system but this doesn’t seem right I can’t even bleed it properly because the motor never shuts off and holds pressure. Thoughts?

 

[AuSeeker]

YouTube

I replaced the whole master cylinder but this one never builds up to full pressure and when you turn the key off it sounds like an air leak maybe internal? I can’t see or feel it anywhere. The seller said it’s doing that because there is air in the system but this doesn’t seem right I can’t even bleed it properly because the motor never shuts off and holds pressure. Thoughts?

Not sure what the hissing could be, but I would suggest buying a mechanic's stethoscope at your local auto parts store for around $10 and you should be able to at least pin point exactly where the hissing is coming from, which may help to determine the cause.