Is that the brake booster? It's an 83 F150. No power assisted anything. Every time the brake pedal is applied the hissing begins. However, there is still a good pedal feel with no sign of brake deterioration. I did swap out the one way valve in the booster but this made no difference. Should I replace my booster?

 

Here is a link to a recent thread where this was discussed. See if it helps?

https://www.ford-trucks.com/forums/1...r-hissing.html

 

Thanks for that. I will order a replacement booster.