I have a 1996 E-150 van. I have occasionally noticed the ABS light comes on, but then goes out after while. I've never had any issue with the brakes. I did replace the pads on the front a while back. Recently, while driving, I noticed that I seemed to have a lot of "pedal" when pressing the brake and, as I sat at a light, I noticed the pedal "creeping" slowing to the floor. None of this affected the stopping however. After getting home I noticed the master cylinder reservoir VERY low. I topped it off and then pumped the brakes, but the pedal still traveled almost to the floor. I can't find any leaks and I noticed that the level is still up where I had topped it off to. I did not that while the engine is off, when pushing the brake pedal it felt normal, but as soon as I started the engine, the pedal dropped considerably. Any ideas? Thanks!

 

Sounds like the booster could be bad.

 

Not sure I'm replying in the right place or not - new to the forum and having some difficulty navigating! Anyway, thanks for the response. Would a bad check valve at the booster or the vacuum line being cracked cause my issue? I'd rather tackle the simple first rather than replacing the booster. Thanks!

 

Something you could do and get an immediate answer.
Remove the two nuts that hold the master cyl to the booster and look for wetness where the brake fluid might be getting sucked into the booster.

 

I’d suspect an internal bleed within the the Master if there’s no evidence of a leak as Hanky mentioned..

 

Thanks, guess I'll start there although there hasn't been any loss of fluid since I topped it off.

 

Fluid loss or not, if you have a sinking brake pedal the master cyl is leaking pressure internally and must be replaced.
If you notice fluid loss after that , now you need to get under the vehicle and check all the brake lines for excessive rust especially where the moisture is held at the tubing clamps that hold the brake line to the chassis.
There is also one other consideration and that is , if the level got so low that air got into the lines you will never be able to get a solid brake pedal, until all the air is bled out of the system.
Boosters don't usually give much trouble, but can't be overlooked.

Something for , FYI, When the level in the master drops down It is a notice of , a leak or time for the pads to be replaced. Adding brake fluid to bring up the level when the pads are worn is not the right thing to do. The problem should be fixed !

 

You can put a dye in the system to find a fluid leak.

 

All of these are possible. Puzzling though - with the engine NOT running, the brakes feel normal - i.e. pedal is up where it should be...you can pump them and it returns to the normal height, you can feel resistance, etc., pedal doesn't sink any. It's only once you start the engine and press the pedal that it drops way down, almost to the floor. No further loss of fluid. Fluid in the reservoir is right where it was after I filled it. No evidence of fluid on the ground or around the wheel cylinders.

 

With the engine Off, and the pedal then depressed a few times it will exhaust the stored vacuum within the booster ‘stacking’ the pedal height on up and making the pedal firm. You basically are then exerting pedal feel again the booster. If there’s trapped air in the master, or, fluid bleeding between the chambers within the master cylinder, the pedal will sink with slight pressures applied after being pumped up. More so an internal leak.
Try this,, engine On, pump the brakes on up and hold ‘hard’ pressure on the pedal. If the pedal remains at normal height after some time,,, then very slowly start to release the pressure. If the pedal now starts to drop with only minimal pressure, good chance it’s a master cylinder internal leak. Leaking internally won’t produce and drips, or leak out the back of the cylinder.
Generally with a small amount of trapped air,, pumping the pedal up and firmly holding it tight won’t allow for the pedal to sink. It’s usually more of a mushy feel to get it to pump up and hold. Air more often causes a generally mushy feel (but will pump up height some) but a internal bleed will most often hold tight until you release a tad of pressure. Internal leaks are fairly common with high service time units..