What's my problem? (old subject, new day, Mustang brakes)

Arthur McDonald · **Joined:** Fri Aug 29, 2003 1:54 am **Posts:** 2052

The MitiVac vs Motive discussion got me thinking about the current brake problem on my 1990 Mustang. It's been a while since I've complained about my brakes, and I figured you were probably starting to worry as a result.

To set the stage, all of the brake system components forward of the firewall except the hard lines have been replaced, some several times, in the last 2 years since the original MC/PB booster pair failed. Now on the 4th replacement MC and the 2nd replacement booster. (Motorcraft has forgotten about this car, so I'm stuck with remans for the most part, though MC #4 was advertised as a new unit and has a rusty iron body instead of the original aluminum body.) This is a stock disk/drum setup, except for a gutted stock proportioning valve and the addition of a manual rear brake bias valve.

Here are the symptoms:

Engine off - a couple of pumps of the brake pedal gives a firm pedal, which does not drop when you keep pressing down on it.

With the engine on, the pedal is spongy when the engine is cold, and until the front pads (Hawk HPS) warm up, stopping performance is pretty weak. With heat in the pads, brake bite improves somewhat. As the engine heats up, the pedal feel becomes firmer, and as has always been the case, the pedal is firmest when you have a lot of vacuum, such as when running higher RPMs and then getting on the brakes.

Here's what's driving me nuts: When I come to a stop and hold the brake pedal to keep the car from moving, the pedal starts to drop. After 20-30 seconds I've reached the secondary and have to pump it again to keep the car from rolling. I'm not losing any fluid. Sounds like the master cylinder is defective. However, as noted earlier, the pedal does not drop when then engine is off and there's no vacuum.

Ideas?

MarcusMcRae · **Posted:** Wed Jun 25, 2008 1:59 pm

Um, you have a vacuum leak.

George Bright · **Posted:** Wed Jun 25, 2008 5:40 pm

It might be a master cylinder still. When the booster is functioning you are putting more pressure on the master cylinder then with foot pressure alone.

Solution sell the track car to a drag racer and buy a new mustang.

Chuck Branscomb · **Posted:** Wed Jun 25, 2008 6:43 pm

George Bright wrote:

Solution sell the track car to a drag racer and buy a new mustang.

Art, don't you already also have a new Mustang?

Arthur McDonald · **Joined:** Fri Aug 29, 2003 1:54 am **Posts:** 2052

Chuck Branscomb wrote:

George Bright wrote:

Solution sell the track car to a drag racer and buy a new mustang.

Art, don't you already also have a new Mustang?

Chuck - Yep.
George - Nope.

Marcus McRae wrote:

Um, you have a vacuum leak.

I've already replaced the hose from the vacuum tree to the booster, so I guess I have to look further upstream. The grommet where the big vacuum line connects to the lower intake manifold is suspect.

MarcusMcRae · **Posted:** Wed Jun 25, 2008 9:29 pm

Arthur McDonald wrote:

Chuck Branscomb wrote:

George Bright wrote:

Solution sell the track car to a drag racer and buy a new mustang.

Art, don't you already also have a new Mustang?

Chuck - Yep.
George - Nope.

Marcus McRae wrote:

Um, you have a vacuum leak.

I've already replaced the hose from the vacuum tree to the booster, so I guess I have to look further upstream. The grommet where the big vacuum line connects to the lower intake manifold is suspect.

Isn't there some kind of check valve between the booster and the manifold?

Vincent Keene · **Posted:** Thu Jun 26, 2008 7:26 am

MarcusMcRae wrote:

Um, you have a vacuum leak.

Technically the amount of vacuum shuld only affect the pedal EFFORT, not the pedal TRAVEL. IOW, with zero help from the booster, the pedal should travel the same amount, but require much more leg effort to do so.

Unfortunately Ryan and I had a similar problem (not exactly the same though) with the Talon and never did figure it out. I believe Ron had a the same problem with his Talon, and to my knowledge never figured it out either.

Good luck Art!

Ryan Holton · **Posted:** Thu Jun 26, 2008 7:57 am

Vincent Keene wrote:

Unfortunately Ryan and I had a similar problem (not exactly the same though) with the Talon and never did figure it out.

Hell yeah I figured it out, I sold the damn car!!

Michael Westerfield · **Posted:** Thu Jun 26, 2008 8:11 am

Ryan Holton wrote:

Vincent Keene wrote:

Unfortunately Ryan and I had a similar problem (not exactly the same though) with the Talon and never did figure it out.

Hell yeah I figured it out, I sold the damn car!!

tomherbert · **Posted:** Thu Jun 26, 2008 8:51 am

My '87 Mustang used for DD and autocross duty developed the same problem. A new master cylinder fixed it but this was 12 years ago when better replacement parts may have been available. I had Hawks on it also and the first stop of the day had to be planned very carefully as there was very little braking available until they got that first little bit of heat into them. Good Luck.

Wes Eargle · **Joined:** Mon Oct 27, 2003 9:47 am **Posts:** 3683

http://www.corner-carvers.com/forums/sh ... php?t=3332

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What's my problem? (old subject, new day, Mustang brakes)

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