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1968 DeVille Brake warning light

Started by rustytractor, April 10, 2015, 04:12:53 PM

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rustytractor

I recently rebuilt all the brakes on my '68 (new flexibles, shoes, wheel cylinders, front cross over pipe, spring kits etc. except for the fronts as spring kits unavailable for '68 with front drums).

The brakes work fine and will of course need to bed in and possibly a re-bleed but since completion I've noticed the red "BRAKE" warning light coming on under hard braking - is this a pressure warning ? It seems that if I pump the brakes then stomp the pedal hard the light doesn't come on.

FYI the master cylinder seems fine and hasn't been touched. I don't have a '68 manual yet so would appreciate a heads-up on what the manual states this brake warning light is trying to tell me.

I can't be 100% positive but I'm almost certain that the light didn't come on before but could be mistaken.

I've mentioned this in another thread but haven't had a conclusive answer so thought I'd post it separately.
Too many cars - too little time !!

54 caddy larry

Sounds like you have air in the system still. Try re-bleeding them and see what happens.
Larry
Larry M

1954 series 62 coupe
1974 Harley Davidson FLH

My caddy is my never ending work in progress.

savemy67

Russell,

I can respond only so far as a '67 is concerned, so please confirm with someone who posts re a '68 (and the '68 manual).  The '67 brake warning light is activated when the brake pedal travel exceeds 2 3/4 inches.  This amount of travel would indicate something is amiss with the system.  Since you posted that you haven't serviced the master cylinder (and the booster I presume), there could be a problem in one of these components.  If you pump then stomp on the pedal, you may be building enough pressure to momentarily keep the brake pedal travel under the 2 3/4 inch limit.  Based on how the '67 system functions, you may want to check the brake pedal travel.  Again, please wait for a post from someone with a '68 manual.  Good luck.

Christopher Winter
Christopher Winter
1967 Sedan DeVille hardtop

rustytractor

#3
I bled them twice when the system was overhauled but will readjust and rebleed again as suggested.

Thanks for the explanation of the warning light - that makes perfect sense.
Too many cars - too little time !!

Glen

The 68 system is different from the description above.  The light comes on because of a difference in pressure between the front and rear systems.  See the attached form the 68 shop manual. 
Glen Houlton CLC #727 
CLCMRC benefactor #104

rustytractor

Quote from: Glen on April 11, 2015, 02:29:51 AM
The 68 system is different from the description above.  The light comes on because of a difference in pressure between the front and rear systems.  See the attached form the 68 shop manual.

Much appreciated Glen.

This morning I checked the old wheel cylinders which came off of the car against the new ones I bought from Rock Autos and noticed that the old rears were 15/16" diameter but the replacements were 1"- it all gets a bit confusing when ordering parts for a '68 due to the switch to front discs part way through the year. Rock Autos product descriptions have zero continuity so whereas one make of item states size dimensions, model, front/rear brake configuration etc in detail another make doesn't so it's easy to make a mistake. Even when you know what to look for it's still a bit of a nightmare because some models with drums or discs at the front are shown as having 13/16" or 15/16" rears whilst others have 1" rears, examples of the problem being:

(With front discs)
AMERICAN REMANUFACTURERS INC. Part # 8436011   
R Wheel/Cyl-Left w/FT DISC/RR DRUM,COMMERCIAL CHASSIS, LIMOUSINE, FUNERAL COACH; FRONT DRUM; 15/16" [Wholesaler Closeout -- 30 Day Warranty] (Only 12 Remaining)

(Without front discs)
AUTOSPECIALTY / KELSEY-HAYES Part # W84003   
Rear; Except Front Disc/Rear Drum; 1" - Except COMMERCIAL CHASSIS [Wholesaler Closeout -- 30 Day Warranty] (Only 1 Remaining)

(With front discs)
NAPA/UNITED Part # 37226   
Rear Wheel Cylinder; 13/16" CALAIS,DEVILLE,62,75 SERIES,w/FT DISC/RR DRUM [Wholesaler Closeout -- 30 Day Warranty] (Only 8 Remaining)

(No front brake configuration or bore size specified)
ACDELCO Part # 18E743    {#19109464} Professional Durastop
Rear Left; Coupe; Sedan

See the problem ? I've never encountered this problem before with any other Cadillac and this one is becoming a bit of a mission.

The 1/6" bore variance may be the cause of the pressure differential which is why the warning light comes on but I'm still unsure exactly what my car should have - the rears have no doubt been changed at some point in the past 47 years and the wrong size may have been fitted.

I still intend to re-adjust and re-bleed the system early next week but can anyone tell me with 100% certainty what size cylinders would have been fitted both front and rear to a '68 DeVille with drums all round ?

As previously stated, I'm fairly certain that the light didn't come on with the old rear 15/16" cylinders but I may just not have noticed.

Oh, by the way the fronts were (and still are) 1.1875 In. Bore (or 1-3/16" Bore depending on which Rock Autos item you look at !)
Too many cars - too little time !!

Scot Minesinger

Was the  rear steel line from proportion valve to above rear axel replaced?  Was the rubber rear brake line replaced.  Old fluid can crust, internal steel lines can rust from inside and create a blockage, which may create differential pressure between front and back brakes.  Maybe the work in front (new cross over tube) cleared the lines a little making more pressure to front, which created the greater differential and energizing the light.  If you did not replace those rear lines (steel and rubber) I would do it.  The new ones have a copper colored alloy to prevent rust on inside and are super nice. 
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

m-mman

Yes the light is illuminated whenever there is a difference in pressure between BOTH SIDES OF THE MASTER CYLINDER.

Everybody loves the dual master cylinders but too few understand how they work. Two reservoirs each operates two wheels. Most American manufacturers set it as front and rear but some European manufacturers set them up as each operating one front and one rear. (diagonal)

Each side of the master sends fluid to the "pressure differential switch". The PDS is a spool valve that can slide one way or the other if the pressure is not equal. As it slides it trips an electrical switch and turns on the light. GM cars had springs that RECENTER the spool valve after all pressure is released and then the light goes out.  In any case as long as the PRESSURE is the same, the the light will not come on.

So what creates situations where pressure is UNequal?

#1 A leak - the main reason for the dual master and the PDS. Pop a rubber line or blow a wheel cylinder and you will still have some brakes but the light will come on to tell you to get service.

#2 Air in one side of the system - Bleed it good!
Are you getting good clean fluid at EACH WHEEL? Dont use any automatic or 'one man' bleeder tool. Get your wife/girlfriend to pump the pedal and then HOLD IT as you open the bleeder screw at each wheel. (she will feel the pedal fall to the floor as this happens) You will be watching for a strong steady stream of fluid squirt from each wheel. If there is no air it will squirt fluid continuously until her foot hits the floor. (have her keep it there until you can close the bleeder)  Did you get some fluid and then a "flatulence" of air, maybe followed by more fluid?  Then have her pump it and open it the bleeder again. (of course checking the fluid level in the master cylinder after each bleed)

Repeat until you have a solid stream of clean fluid at each wheel. (this allows you to verify that fluid IS ACTUALLY COMING to each wheel too)

#3 The Master cylinder is bad. Yup, a dual master is actually TWO master cylinders in one. If just one of the rubber cups is leaking (however slowly) then the pressures will not be equal.
Of course since you have done everything BUT the master cylinder . . . . ???  The question must be asked how you determined that it was 'fine' when everything else needed replacing? . . . . Dont worry we have all been there . . . .

Note to old car owners: Brakes are really important. Replace everything. (especially anything with rubber in it) it is never worth it to skimp in restoring a brake system.

Personal rant: People complain about old brake systems (and they were never as good as modern brakes) but they complain before they have even correctly fixed the original components. 

My thoughts - replace the master cylinder - give it a GOOD BLEED  and you will be fine.

As for the different sized wheel cylinders?
Dont get worried. You want them to be the same for each side but if they are a tad bigger that is actually OK.

The Master cylinder is sending fluid pressure to the back of the piston in the wheel cylinder. The pressure (force?) that the wheel cylinder exerts against the shoe is related to the size of the piston. The bigger the piston, the more force. . . .

Just like in the engine. Allowing for identical compression ratios, the exploding gas charge is (effectively) the same in all engines however the one with the bigger bore gives more power. Bigger wheel cylinder? More force on the shoes.

In some years Cadillac added a slightly bigger wheel cylinder to the Commercial Chassis to compensate for the added weight despite using shoes that were identical to the passenger car.
1929 341B Town Sedan
1971 Miller-Meteor Lifeliner ambulance
Other non-Cadillac cars
Near Los Angeles, California

CLC #29634