89 240 DL wagon- brake system failure light came on today- fluid level looks fine, manual says one of the brake systems is not functioning. Luckily I'm only 6 miles from home. Any thoughts? Thanks! Dave

 

probably an air bubble in the distribution block.

 

Adub- Would that be a matter of finding that block and tapping on it (to move the bubble) and bleed the lines out? I bled them (by the book) about a month and a half ago, but they never felt quite right. Dave

 

Can anyone explain the workings of the circuit? is it a delta/P switch in the system? how do it know if one channel recieves less pressure?

 

Yes pretty much, it should be directly below the master cylinder

 

Would it not be better to pressure bleed the brakes from the calipers to the master cylinder? air has a better chance to move upward than to be forced through the system. power brake system on the aircraft I fly and maintain

 

I've seen power bleeders that force fluid through the lines toward the calipers and ones that suck it from the caliper bleeder but never one that goes the other way. It has to do with the crud in the calipers at the bottom of the system. Back flushing would send it back up. In ABS cars, that can prove disasterous.

 

I've never had any problems bleeding brakes the conventional way, using teh master cylinder as a pump.
I seriously doubt the brake warning light has come on due to an air bubble, probably a proper fault in the system or a defective switch. The engineers allow for things like air bubbles and make teh systems accordingly.

Regards, andrew.

 

She's been sitting since last week, went out of town. Guess I'll try re-bleeding tonight as my first course of action. All 4 of the calipers are only about a month old. and my fluid level looks fine- pedal does go pretty far down though...Dave

 

leak in diaphragm of power booster?

 

I'm going to post up my PM reply to Dave here, seems useful information for the forum.

To verify a leaking master cylinder, proceed as follows.
With ignition off, press brake pedal several times to destroy vacuum in the brake booster. You will know when you've achieved this, the brake pedal will go very hard.
Now, push fairly firmly on the pedal with both feet, around teh sort of pressure you'd use doing a hard stop.
Hold teh pressure constant and note if the pedal moves slowly to the floor. If the pedal moves, you need a master cylinder rebuild/replacement.
If teh pedal doesn't move, I would suggest a thorough bleeding of the brake system, following the relevant instructions for your car (split brake systems bleed differently to normal and ABS systems).
If you still have s soft pedal, have someone push the brake pedal hard whilst you look at the rubber hoses at teh calipers, significant growth of the hoses means time for replacement.
Have fun, brake problems are usually pretty straightforward.

Regards, Andrew.