Can't get power brakes to work

-
crewchief

crewchief

A & P Mechanic
FABO Gold Member
Joined
Jan 17, 2014
Messages
656
Reaction score
365
Location
Malta NY
Equipped with factory front disc and 10 inch rears all pads/rotors and shoes/drums replaced. I've replaced the booster (Midland-Ross) and check valve and still have no power braking action. When this car was new, they worked beautifully. Vacuum is at 18-20 and rear brakes adjusted. Any hints would be helpful.
 
crewchief said:
Equipped with factory front disc and 10 inch rears all pads/rotors and shoes/drums replaced. I've replaced the booster (Midland-Ross) and check valve and still have no power braking action. When this car was new, they worked beautifully. Vacuum is at 18-20 and rear brakes adjusted. Any hints would be helpful.
Click to expand...

If everything is new, then it must be the booster.
That is unless you mean the brakes don't work but the booster does.
If you put your foot on the brake and then start the car does the pedal go down farther?
 
Yup, pump 3 or 4 times key off. With foot on pedal, start car. Pedal should drop 1". That is the booster test.
 
If I recall correctly, the pushrod that comes out of the booster into the master cylinder has an adjustment. I don't know the spec off the top of my head. Is there a hex drive on that piston from the booster? I would think the spec would be in the FSM.
 
Brakes work just fine but lacking the assist of the power brake. It's like the power brakes aren't there (option missing). I can pump up the brakes, start engine and get pedal drop. Power brakes just are not there.
 
I've owned this car since it was 2 1/2 years old. It had great working power brakes until I took it off the road back in the mid eighties. Now restored, it just doesn't have the stoping power it once had. The booster was replaced as well as the check valve. Good vacuum and brakes, just not sure what's going on.
 
The pedal drop is the extra push from the booster. So now its a matter of what else could be malfunctioning. If everyting is new/near new then whats left?
Master cylinder bore incorrect? Its not uncommon that an incorrect bore master is sold as a correct unit. Diameter is different from power to manual. But generally power goes up a size.

Upon replacement of all the new parts, bleeding process seemed normal?
 
Tooljunkie said:
The pedal drop is the extra push from the booster. So now its a matter of what else could be malfunctioning. If everyting is new/near new then whats left?
Master cylinder bore incorrect? Its not uncommon that an incorrect bore master is sold as a correct unit. Diameter is different from power to manual. But generally power goes up a size.

Upon replacement of all the new parts, bleeding process seemed normal?
Click to expand...
Master cylinder is original equipment, system was bled after new lines were installed. Don't believe the master cylinder could be the culprit here. Only booster was replaced along with valve.
 
I wouldn't hold much hope for a master cylinder that old. I would expect wheel cylinders, calipers, everything with brake fluid and old rubbers in it to fail, even if they worked and didn't leak initially.
Then there's the possibility of a collapsed hose or partially clogged port starving the booster. A vacuum gauge would show the same on a much smaller line.
 
crewchief said:
... I can pump up the brakes ...
Click to expand...
What does this mean? You don't need to pump the brakes. You should have a fairly stiff pedal all the time, even with engine off. Is there air in the system?

You can test the booster by itself. Hook it to a vacuum source, like your engine running or even another car's intake vacuum. As you push in the pedal rod, the output rod should move with very little resistance. The only force needed is to overcome the weak spring on the booster's inlet valve. You need to adust the tip on the output rod so it just barely touches the MC piston, i.e. minimal initial displacement but no "lost motion" gap. You can feel for that as you bolt the MC up.
 
BillGrissom said:
What does this mean? You don't need to pump the brakes. You should have a fairly stiff pedal all the time, even with engine off. Is there air in the system?

You can test the booster by itself. Hook it to a vacuum source, like your engine running or even another car's intake vacuum. As you push in the pedal rod, the output rod should move with very little resistance. The only force needed is to overcome the weak spring on the booster's inlet valve. You need to adust the tip on the output rod so it just barely touches the MC piston, i.e. minimal initial displacement but no "lost motion" gap. You can feel for that as you bolt the MC up.
Click to expand...
Adjusting the output rod is done how?
 
I don't know what the FSM says, but I just screw the adjustable tip outward, then offer the MC to the booster until I feel the rod touch the piston and note the gap. I screw the rod in a little less than that and repeat until I can feel it touch with just ~1/16" gap. That gives slight piston displacement as you bolt it up (so tight and no lost motion), but not enough that the piston will not still expose the refill port when the pedal is up.
 
You must log in or register to reply here.
-

Back
Top