Brake Failure Warning Light

What would cause the "Brake Failure" warning light to come on after driving for 30 - 40 minutes and a good and warm engine? The brakes still seem to work OK.
This happened a couple years ago so I rebuilt the calipers, installed new pads and bled the system 4 times (had a ***** of a time getting all the air out of the rears). I even installed a new brake pressure sending unit.
Everything seemed fine until this Spring when the light started coming on as described above.
I probably should have, but didn't, rebuild the master cylinder and the rear proportioning valve. Could these be the source of the problem? I'll re-bleed system for a start - maybe that's all it'll take. Wishful thinking??
Any comments/suggestions will be much appreciated!
Bill

 

Usually the proportion valve is the problem. To test it you need to pull the electrical connector on the valve assembly and drive it around. If the light stays out then the valve is the problem. Also I assume you checked the level floats in the brake fluid container to make sure they didn't "sink" and trigger the light. Low fluid is the same circuit as the proportion valve. Those are the only 2 places that control this circuit.

 

Check your vacuum hoses. I had the wire internal support rust leaving a sharp edge that cut through the hose. No vacuum, no brakes. Good idea to just change out the hose if it is original. Easy to pull through the lower rocker area. Remove left front wheel, remove rear shield panel. Open hood, remove plastic moulding at rear of tire recess so you can see vacuum booster. Tie a stout string to the engine end of the vacuum hose and pull the hose forward into the wheel well. String will follow its path. Then pull the hose into the front compartment. Cut a short piece of new hose and run between carb manifold and vacuum check valve. Make sure check valve is oriented correctly. Attach new hose to engine end of string and pull hose forward. Attach hose to check valve and make sure it fits nicely behind left tank. Go back to front, pull hose and route loosely to booster. Cut to length and attach new hose to booster. Put everything back together AFTER you have tried things.

Vacuum hose is available in most auto supply stores. 14 mm is nominal ID. Wall is generally at least 1/4" thick. Make sure that it is VACUUM hose that you buy. Wire reinforcement is not necessary with modern hoses. Good luck.

John

 

Thanks for the info Steve & John!
The brake fluid res. are full so that doesn't appear to be the problem. Other than the caliper rebuild and replacing the pres. sender, I've done nothing to the brake sys. in the 4 years I've had the Dino. It only had 28K miles when I got it but sat in storage for 20 +/- years. Maybe it's time to R&R the whole sys., i.e., master cylinder, proportioning valve, distribution blocks, steel and rubber lines, etc. I'm buried in another resto project now so, other than a quick fix, the Dino brake issue may have wait.
Bill