Brake Pedal "pushes through" while waiting in traffic?

[screenwriter-X]

I can't seem to find this issue in searches, so I thought I'd explain it and ask for suggestions.

I drive a 1989 3.2 and in the past four weeks I put into a temporary commute in rush hour traffic. Lots of waiting around. Sucks. Luckily, that's over now, but an issue came up with the brakes that happened twice.

I replaced/rebuilt the brakes two years ago myself, returning them to stock, and they have been fantastic. Four years ago (and not that many miles ago) the master cylinder was replaced just before I bought the car. So everything has been terrific in the brake department.

However, after a few weeks of the rush hour commutes, I noticed in traffic an odd thing. Of course the brake pedal has been nice and firm with great feel and modulation, but in stop and start traffic, suddenly the firm brake pedal gave way and the car rolled ahead (downhill). As I put more pressure, the brakes came back and stopped the car before I bumped the into the person ahead. I was concerned for the rest of the drive home, but the problem went away after repeating only once more. A few days later, the same thing happened, then went away again.

After the first incident, I immediately checked the brake reservoir and it was topped up. And everything worked great just yesterday, so I'm at a loss as to what it could be. Is it perhaps some kind of electrical issue? Is it a flag that I should replace a component before it fails permanently?

[tdskip]

I believe that is indicative of the booster going south - get other opinions before doing anything about it.

[s_morrison57]

Just a thought cause I'm no expert but maybe its air in the system, have you tried to bleed the brakes, cold they work good - hot maybe not so well ?????
Finn

[AVDCAV]

Look at the slave cylinder for the clutch " under the
Pedal board . They leak And also make a mess.

[village idiot]

Most times if there is a loss of pressure with no external sign of a leak, the problem will be the master cylinder.

[chrismorse]

If the pedal creeps to the floor, particularly on light application, like at a stop light, then fluid is leaking past the cup seal.
If you mash the pedal, the fluid pressure is enough to hold the seal lips against the bore and the car stops just fine. With light pressure, fluid can seep past the lip and the pedal creeps to the floor.

This symptom is evident acros the car spectrum.

Air would just give you a consistent spongy pedal.

Changing the fluid every year should prevent moisture accumulation in the system and subsequent bore rust and seal failure.

HTH,
chris

[tdskip]

Does the pedal sink with your foot on it at a stop? Will the pressure come back up if you pump it rapidly?

[porsche930dude]

check the vacume line for leaks . works best with a fog machine .

[gregwils]

Originally Posted by porsche930dude

check the vacume line for leaks . works best with a fog machine .

+1

vacuum leak you probably didn't ever think of

[Joeaksa]

Originally Posted by village idiot

Most times if there is a loss of pressure with no external sign of a leak, the problem will be the master cylinder.

Bingo!

[screenwriter-X]

Thanks for the replies. I will look into the vacume line first. Brakes worked perfectly this morning on a test drive, stopping fine with light pressure and firm pressure and while waiting at a stop light, and there's no external leaks anywhere or loss of fluid. I've only experienced the problem while waiting for long periods idling in traffic jams with my foot on the pedal for a very long period so I don't roll downhill. It's difficult to accept that the master cylinder would go after only four years and 6000 miles of street/highway driving while getting regular flushings, so hopefully it's just a vacume line thing.

[bfunke]

I'd look at the MC first. Vacuum leaks usely creat an intermittent loss of power brake with resultant hard pedal requiring a lot of foot pressure. Sinking pedal is usually a leak at the cup seals. Pull your pedal board and see if you have any leakage. If the MC is bad you can either rebuild or replace. Brak fluid absorbs moisture over time and that causes rust in the MC and slave cylinders which causes small pits in the bore which cut the seals and also allow fluid to pass. If the pitting is minor you can clean it up with a brake hone and electric drill using brake fluid as honing solution. Be sure to run the hone up and down the bore looking for that nice crosshatch pattern just like honing a cylinder.

[bfunke]

PS - this is why you should change out your brake fluid every 1-2 years.

[RWebb]

test the pedal "push down over time" with the engine off

[Walt Fricke]

Despite the presence of the vacuum booster, there is a completely mechanical link between the brake pedal and the first part inside the MC. Which is why several responding are pointing you to internal leakage within the MC despite the fact that the MC is fairly new.

As you depress the pedal, the system also opens up a valve to admit vacuum to the booster pie plate. Because the diaphragm is pretty large, a small vacuum can produce quite a lot of force. The farther the pedal is depressed, the more this valve opens up and the more boost it gives to what your leg and foot are producing in the way of force.

A vacuum leak would produce less pressure from the same foot pressure, not a pedal which lowers to maintain the same pressure, would it not? I suppose your foot might move back (as you maintain the same pressure with it) as the booster does less and less? I'd think it would take quite a slope for this to allow the car to move, though.

External leakage from the rear of the MC on a boosted car usually ends up inside the booster, doesn't it? So you only discover this early by pulling the booster? However, if your reservoir is still as full as can be, unlikely you had an external leak. Internal leakage will not lower the reservoir level.

In the older cars with the MC down by the pedals, external leaks go into the rubber accordion boot, and in time out into the pedal cluster and floorpan.

[bfunke]

If it was made in China it's probably shiite and destined to fail anyways. I just replaced a radiator in my Tundra and had to install 3 new ones before I found one that didn't leak. Arggh.