I just did the rear brakes my truck. I also upgraded to the 37337 wheel cyl. So bleeding the brakes was needed. Bled them with the engine off. After the job was done I was doing some testing and noticed something. When I step down hard onto the brake pedal when the engine is running, it sounds like an air/vacuum leak. It doesn't do this until I hit the last 15% - 20% of brake pedal travel. Is this my hydro boost unit going bad maybe?

 

hi
no air or vac in the hydro boost .

how is your power stearing / hydroboost fluid level ?

maby sucking air in pump?

 

I would check the master cylinder because what happens is after pressing on the brake pedal for years on end, the master cylinders plunger usually only travels so far in either direction. But because brake fluid only moves a minimal amount, crud, corrosion, etc will collect at the end of the plungers sweep. This is never a problem until the one day you decide to bleed your brakes or change the brake pads. While bleeding the brakes or depressing the brakes to reset the calipers, you're having to use the full sweep of the master cylinders plunger which forces the plunger into all that crud, damaging the plunger. This is the reason its often seen that when people replace brake pads/shoes, they end up having to replace the master cylinder too. Not saying that this is your problem either since I have no idea what the noise is you're referring to.

 

I'd suspect its normal, with brakes applied that hard the Pump and hydroboost are maxed out