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1970 Cadillac RWD power brakes do not work-it was a bad power booster

Started by Scot Minesinger, December 31, 2014, 12:23:04 AM

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Scot Minesinger

The shop manual is not much help here I do not think.

I was driving and everything was great, and wow next stop took tremendous effort on brake pedal.  The brakes worked, just no power assist.  Using two feet and pushing super hard, the car stops from 50 mph OK, although I'm not strong enough to apply enough pressure on pedal for a fast stop/lock brakes up.

I checked the brake booster and it holds vacuum, pumped it up to 10 inches after many squeezes on vacuum pump, and it held for an hour.  The check valve on booster holds vacuum.  Measured vacuum on carb at power brake booster connection and it gets 20 inches easy.

Brake fluid is full, not leaking.

The brake booster and master cylinder were replaced 4 years ago from NAPA, and have maybe 15k miles on them or so.

The shop manual suggests vacuum leak, bad pistons, leaking brake lines, glazed linings, and other causes that seem improbable.  Since the failure was not gradual, but immediate, I think that rules out glazed linings and other things that take time to occur and cause the problem.

Is it a bad booster/master cylinder?  Hate to replace a relatively new unit and it not resolve the problem.

Thanks,

Scot
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

The Tassie Devil(le)

Sounds like the Diaphragm is suspect.

When you start the engine, with the foot on the brake pedal, does it descend to the floor as the vacuum takes effect?

When you stop the engine, and operate the brake pedal, does it work as normal, then after a couple of pumps, go hard?

If there is no vacuum there, look around the carby for a loose hose to the Booster.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

Scot Minesinger

The brake pedal is hard to push down when car is running and off.  The vacuum hose at carb is perfect.  It is like the booster is not operational, but the brakes work with great effort.  I'm thinking of replacing booster and master as a unit.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

TJ Hopland

You should be able to pop the check valve out of the booster, see if that is maybe jammed up.   If you got vacuum to it and it holds vacuum it seems like the vacuum just can't be getting to the correct place.   The check valve is the elbow inlet do dad. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Scot Minesinger

The check valve works, lets vacuum into booster but not out.  It took hundreds of hand pumps to get on vacuum pump to get 10 inches, and when removed vacuum remained.  I just ordered a new booster/master cylinder combo.  Will replace this and keep you posted.  I have never performed brake work before, may inquire later on some ideas.  Plan to use a pressure bleeder, not going top pump brakes, as that is hard on master cylinder I hear.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

cadillactim

Scot

Did you try what Bruce suggested by putting your foot on the brake pedal and starting car, noting if the pedal drops a little? If it does the booster may be okay. If the pedal doesn't drop, then the booster is bad.

There's no need to pressure bleed when you replace the master cylinder. Just bench bleed it before you install it. You're not bleeding the entire system, just the master cylinder and immediate area. Simple job to do. Replacing the booster is the harder job time wise.

Tim
Tim Groves

Scot Minesinger

Tim,

Yes pedal does not drop when car is started.  Very firm brake pedal.  I will be replacing master cylinder and booster as a unit.  How do you bench bleed?  Won't there be an air bubble in the two lines that connect to master cylinder when reconnected?

BTW left voice mail, need to buy some stuff on a 72 Caddy - please call when you get a chance.

Thanks,

Scot
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

"Cadillac Kid" Greg Surfas 15364

Scott,
When all else fails look for the simple solution. Probably 75% of the booster replacements are done because there is a leak somewhere in the vacuum "input" line. Either the carb fitting, the hose, the hose connections, the check valve or it's grommet.  Unless you just want to have fun doing your first booster replacement, from what you said about the booster itself holding vacuum, I would look there first.
As Tim states the Master cylinder is easily "bench" bled.  With fittings in the two discharge ports and short hoses connected to those fittings which are extended to near the bottom of the reservoirs, add fluid and "pump" the piston with a soft tool until the fluid returning to the reservoirs is bubble free.  Then remove the fittings, install temporary plugs and install in the car.  Beats trying to bleed the whole system for nothing.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

The Tassie Devil(le)

Quote from: Scot Minesinger on December 31, 2014, 10:04:20 AM
The check valve works, lets vacuum into booster but not out.  It took hundreds of hand pumps to get on vacuum pump to get 10 inches, and when removed vacuum remained. 
Something wrong here.   The Check Valve is to stop air going into the booster, and let Air be sucked out.   Hence, Vacuum Booster.

There is air pressure on the firewall side of the booster diaphragm, and vacuum on the engine side.   This difference in air pressure on the diaphragm is the way these things work.

There could be a problem with the internal air filter in the passenger compartment side of the booster, and therefore not allowing the air positive/negative pressure doing its' work.

But, whatever it is, it has to be something simple.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

Scot Minesinger

I wish it was something simple.  I checked the vacuum hose, it too is only 4 years old and not leaking.  I have a vacuum gauge on the car and while stepping on brakes it stays at about 20 to 21 inches.  Not anxious to replace a PIA part that I have never done before. 

The check valve lets vacuum into booster and not out.  That means it lets air out, but not in.  It is working.  With hose disconnected at carb and connected to a vacuum pump, it holds vacuum for hours.  With gauge on carb port supplying vacuum to booster it reads 21 inches.  So carb makes vacuum, hose, grommet, and check are all good.

Plus the brakes stop the car well just requires intense leg effort. 

Thanks for the bench bleed technique, will do that.

On the bright side, pun intended, found the source with the flickering lights and it is cured.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Jeff Rosansky CLC #28373

Do you think you have a blockage in one of the brake fluid lines?  Or maybe the check valve (where the brake light switch connects).  I would bleed one of the rear wheels and see what the fluid coming out looks like.  Mine was basically dry (I had no rear brakes) and the crap that finally came out was like maple syrup.  If that check valve is plugged then it would give you a very hard brake leading you to think it is the booster where it really isn't. 
Jeff
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

Scot Minesinger

Jeff,

The check valve is not plugged because it lets vacuum in but not out.  I tested it and hose first thing.  I replaced all the brake lines with new steel lines and all three of the rubber flex lines four years ago to avoid problems you describe.  The brakes work great, it is the no power assist that is not there for me. 

Many think as you that it might not be the booster, and all indications are that it is.  The reason I posted this was because my experience with vacuum devices is that when they fail they don't hold vacuum, but apparently in internal failure in the rubber disk in the booster does not release vacuum and car still runs well.

The other thing is that the power feature of the brakes failed suddenly.  A build up in brake lines, and many other possibilities mentioned in shop manual for firm brake pedal are things that happen over time.

Thanks and happy new year!

Scot 
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Jeff Rosansky CLC #28373

Actually Scot, I used he wrong term-my error.  The book calls it the distributor switch. But now that I think about it, that is a 2 part sealed switch so if 1 side (front/back) was plugged it would turn the idiot light on.  So, forget that--Sorry.
Jeff

May I suggest disconnecting the vacuum line and driving the car.  The reason I suggest this is that you can see if the "feel" is the same as what you experienced.  If the feel is different then you know it isn't a booster issue.  That might help you determine if the booster is your gremlin or not (I know it isn't an AMC).
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

Scot Minesinger

Jeff,

Yes I did drive car with booster vacuum hose disconnect, car drove exact same.  Had vacuum gauge on carb port that connects to booster to see that I had vacuum.

Yes, had to cleanout that brass proportion valve too to keep brake warning light from turning on.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

dplotkin

Scott: With all due respect to my fellows here who are giving advice, change the booster and it will be fine. Boosters can easily fail and still hold vacuum, where folks got the idea that's impossible I'll never know. Boosters can fail all kinds of ways, including working perfect except for extended pedal travel, a case I had on my 62 Bonneville.

Boosters are not expensive, are not terrible to change (unless you lack physical alacrity - it's far less challenging than a heater core) and above all, boosters are perishable items. They fail with time and use. They should be changed together with the master cylinder as a leaking rear seal is often why the booster fails to begin with.

Don't mess around with brakes looking for a hard to find cause. If it's simple and obvious, fix it. If not, change the assembly.

Dan
56 Fleetwood Sixty Special (Starlight silver over Dawn Grey)
60 Buick Electra six window
60 Chrysler 300 F Coupe
61 Plymouth Savoy Ram Inducted 413 Superstock
62 Pontiac Bonneville Vista
63 Chevy Impala convertable
63 Ford Galaxie XL fastback
65 Corvette convertable 396
68 Chrysler New Yorker

Scot Minesinger

Dan,

That is exactly what I will do - change the booster.  It should arrive tomorrow, will paint it golden CAD (Eastwood), if it is already golden Cad will clear coat 
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Scot Minesinger

Replaced the power brake booster, bench bleeding the master cylinder and then cracking the fittings with power pressure brake bleeder.  Then drove on my quarter mile unsalted court and the brakes seemed fine.  Need rain to clear salt and then can test drive.  To all those who suggested replacing the booster that seemed logical after checking everything else - thanks.  Picture of power brake pressure booster in action.  You can bleed brakes with one person and you do not have to engage the master cylinder thru brake pedal.


Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

Jeff Rosansky CLC #28373

I know I have said this before, but I will say it again.
I believe the reason your old unit failed, is that it is simply too clean under your hood.  I mean even the wrench and socket are clean.  Dirt helps hold things together!!

Seriously, good job.  I have heard of those bleeders but have always just done them the old fashioned way.
Jeff
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

Scot Minesinger

Jeff,

The dirt is there, just ask my wife, she complains about how my hands do not wash clean for days after working on cars.  Those chrome finished tools wipe clean and always look good, no dirty tools with that chrome finish.  With a fresh engine re-build, your car will look better than mine, as engine in my car is on a ten year old 35k re-build.  On to a few minor things to correct:  adjust NS switch, adjust top convrt rail, lubricate behind rear drum shoes (underside), and repair e-brakes.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

TJ Hopland

So you like that bleeder tool?    That is the same one I have.    Didn't it make a mess when you unhooked it from the mc?   I tried mine on 3 or 4 different cars with limited success and a big mess.  Its been several years since I have seen it but I know its out there somewhere.  I bought the deluxe kit that came with like 6 different caps.  Figured maybe someday I would need to do a clutch or something.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason