Hi all,

I've never had a master brake cylinder go bad before, so I need help diagnosing it before I go and spend $90 on a new unit.

I've bled the f**k out of the brakes and can't get the pedal to firm up. There's lots of boost, but the pedal just goes to the floor. While driving around the neighborhood, I can get at least one rear wheel to lock up but not the front. The pedal does not firm up even if I "pump" it. There are no fluid leaks anywhere, and all the brake lines appear to be in good shape.

Is a new master brake cylinder in my immediate future?

Thanks,
Matt

Sure sounds as though you need a new MC. Pedal-to-the-floor with no braking action is telling evidence.

If when you pumped the brakes the pedal DID firm up, would that still be the master cylinder?

When an mc goes bad, it is usually b/c the piston cups no longer seal to the cylinder walls and fluid just goes on around them rather than being pressurized.

I am a 240 guy, but this may work on 740. When you are stopped, engine running and in neutral, hold the car in place with minimum brake pedal pressure. Then goose the engine and let it drop back to idle. If the pedal seems drawn to the floor as the engine drops to idle, it is the mc, period, end of story.

Good Luck,

Bob

:>)

Yes, and don´t try to renovate it. Even if it looks good, there is corrosion/wear on the surfaces, so it is just not enough to replace the seals. Get a new one, complete, or try some from the junk yard if you have time and patience. This coorosion is why one should flush brake system every year or two. (don´t ask, I have been lazy!)

Gustaf