I am starting to question the brakes on my 2000 Expedition with 47,000 miles. When I hit the brakes hard, there is a slight air rushing sound, and if I put a lot of pulsating continuous pressure on the brake pedal it will sink to the floor. Am I looking at a master cylinder or vacuum booster or both? How do I diagnose one or the other, any ideas?

 

ever had the brakes done...47 on a expy is about the range...also when you do it have a flush and bleed done to the hydraulics

 

A visual inspection of the front pads shows plenty of pad left. When it warms up a bit here in the NorthEast I will flush, fill, and bleed the system. But, I really need to know how to diagnose a Master Cylinder or Vacuum Booster problem.

 

In my past experiences with a bad master cylinder I noticed that the pedal would feel soft sometimes. And several times I was sitting at a light with my foot lightly on the brake pedal and it would suddenly sink to the floor. If I took my foot off the pedal then applied it again the pedal returned to the normal height. That happens because the brake fuid is slipping past the seals in the master cylinder.

I've only had one bad booster and it was on an old mustang. In this situation the booster worked intermittently. I would be driving along and when I started to press on the pedal it felt like manual brakes. Several times I had my foot on the pedal and the booster began working properly all of a sudden while coming to a stop, this caused me to screech the tires and lock them up while trying to stop! It was then that I realized how little pedal effort is needed w/ power brakes as opposed to manual brakes ..... I also realized that I nearly cr@pped myself.

When I replaced the booster on the mustang it came with a master cylinder bolted to it. The two came together as a unit.

This only my experience and you would be wise to check the manual for your truck for a proper diagnosis. Brakes are no joke to be doing half@$$ed.

Good luck.

 

A vacuum booster won't allow the pedal to drop to the floor like you describe. It simply uses the vacuum from the engine intale manifold to reduce the foot force necessary to brake at a given decel level. If it's not working you'll experience a very hard pedal.

If the pedal drops to the floor as you describe, it's one of 2 maybe 3 things...First, if you haven't already done so, bleed the brakes, you may have air in the lines. If that doesn't work (and you have drum brakes), check & adjust the rear drums. If both of those fail, it probably is the master cylinder.

If, after doing both of the above, when your pedal drops to the floor, the 'brake' light on the dash should come on indicating a pressure differential between the front and rear brake systems. Since you've just eliminated the 2 other reasonable causes for a pedal drop/pressure differential, 10 to 1 it's the master cylinder.

Good luck!

Tom

 

Thanks 8mmag

Thats exactly what I was looking for. So it looks like my vacuum booster is doing exactly what it is supposed to. When it warms up a bit here in the NorthEast, I will drop on some Russell Performance brake lines, flush, fill and bleed.

Thanks again