k, so I've got a 1990 honda accord, and I can put the brake pedal to the ground almost. And I'm not talking about "squishy spongey please bleed me" brakes, I'm talking smooth ride to a dead stop almost. I'm not losing any fluid anywhere, and when I step on the brake pedal I can hear a hissing sound. Took it to a local mechanic, and he looked it over and said it's probably my brake booster. I asked if it was the master cylinder, and he said more than likely no, since no fluid was being lost, etc etc. I was just wanting to get a second opinion...or 20, since ya gotta <3 the internetz, on whether it actually has a high possibility of being the booster before I run off to the junkyard to pick one up, since he barely took the time to give a decent one over?

 

Booster

 

booster. that hissing sound you hear is a seal gone bad which is why the pedal goes down to the floor

 

Sorry, I vote master. They can and do fail internally meaning no leaking, just no pressure being applied to the calipers/wheel cyl.

The booster is a vac assist which make the force needed on the brake pedal less. Get the car in an empty parking lot, get it up to 20 mph, turn the key off - DON'T LOCK THE STEER WHEEL - apply the brakes. See how much more force is needed to stop the car.

Again just my vote.

 

But master cylinders don't hiss !

 

Try bleeding the brakes first. That should be the first thing you should be doing assuming there are no leaks.

 

You are missing the point. You DO NOT need a brake booster for the car to stop and the brake pedal to remain at a "proper position". The main reason for the brake pedal to "sink" to the floor - air in the system, thanks holmes for asking the op to double check - is that the master is bad.

Think about it this way....your car will still turn even though the PS pump is bad............have a good day