Brake problems - booster?

We have a 2001 Mercury Villager with ABS and about 103,000 miles. The brakes have started to have an intermittent problem. Initially when my wife backs out of the garage in the morning the brakes are fine with the first push of the pedal, but the second time that she pushes on the pedal it only goes part way down with very little braking effect. Saturday she told me that she drove through two stop signs because it has started to have this symptom when she is driving it and the car is warmed up.

The Haynes manual says that with the engine off the pedal should depress to the same point when you press it three times in a row. It doesn’t, it pumps up. Unfortunately Clymer doesn’t say what that would indicate. Also Clymer says that if you depress the pedal and then start the car, it will depress further after the car starts. When its cold that takes about 10 seconds, when the car is warm the additional motion is immediate. Since the minimum boost problem is intermittent I am reluctant to take it to a shop, on the other hand I don’t know how many more stops my wife can miss and stay alive.

I’d say that sounds like a bad brake booster. Check out the vacuum line and the check valve (check the check valve?) if it has one. If those two pass, it’s booster time.

The highest vacuum is at idle. The check valve keeps the vacuum in the booster during acceleration and the size of the booster is a design feature to maintain enough vacuum through several brake uses. The check valve is usually the hose connection at the booster. Removing the hose, you might be able to hear it leak. A couple buck item, you might as well replace it.

Also I would have a look at the vacuum canister. The purpose is to store vacuum and if you have a crack or leaky fitting or hose, you could be starting out with no stored vacuum in the morning.

Here’s your test for the booster. With the engine off, you depress the brakes three times. The first time it goes the closest to the floor, about half way normally. Each of the next two times, it doesn’t go quite as far down as the previous time. After the third time, press and hold the brake pedal, turn on the engine. It should go immediately down to the first position.

I think that you may have a problem with the master cylinder or ABS but at this point, thats just a WAG. You need to find a good mechanic before this ends up costing you a lot more.

I am suspicious about the delay in the additional pedal travel when starting the engine cold. Do you know what is happening mechanically/pneumatically when after starting the engine, the pedal has the additional motion?
It occurred to me that I could replace the booster, check valve and master cylinder for less than one trip to the shop. I think that about $150 to $175 would replace all of those components. ABS seems to be working OK.

Would you believe that the check valve is a dealer only item and is $45? Hope that it helps - but its still the cheapest component. If I’m tearing into it that far I’m replacing parts regardless.