Friday, August 3, 2012

Smoking brakes and Crewe era Bentley and Rolls-Royce hydraulics



The Rolls-Royce in the photo above is a prize-winner, as the one in this story.  Just not the night it failed to proceed.  Imagine the scene . . . .

You’re driving down the highway, thinking all is well.  Gradually, you come to notice a vibration.  You look in back, and see smoke.  Applying the brakes, the pedal feels funny; like it goes to the floor.  The brake warning light comes on.  The car stops, and you wonder what went wrong.  Smoke comes from one rear wheel arch.

Your motorcar assumes the British Position, on the back of a car carrier, and you find a ride home.

Later, your car arrives at the shop.  When the tow truck drops it off, it looks and runs fine.  No smoking, no brake drag.  Brake fluid is full.  There are no obvious problems.  They put he car on the lift and see a trace of grease on the inside of the rear brake rotor.  Did it come out of the hub?  Maybe.

Sound familiar?  Many owners of 1965-1980 Silver Shadow cars have experienced this, or something similar.  What happened here?

I had this happen, here at Robison Service, last week.  I’ve described how we figured out the failure, both because it may happen to you, and the process may help understand how we solve problems with these cars complex hydraulics.

We realized one of two things must have happened:
  • ·      The wheel bearing could have failed and overheated the area
  • ·      The brake could have dragged and overheated the rotor and hub


Either failure would account for all the symptoms
  • ·      The overheated parts would smoke
  • ·      The hot rotor would vibrate
  • ·      If the rotor got hot enough, it would boil the brake fluid, which would make the pedal go to the floor, and light the lamp
  • ·      If the hub got hot enough, grease would melt out of the bearing and perhaps remain on the brake disc.
  • ·      There would be no loss of fluid and it would look fine when cool.


The wheel bearing is a simple failure, whose cause would be visible in the hub.  A brake drag could be caused by several things in an early Shadow
  • ·      The caliper pistons could be frozen
  • ·      Pads could be rusted in the caliper
  • ·      The conventional master cylinder to the rear wheels could have stuck
  • ·      One of the distribution valves might have leaked
  • ·      The brake hoses might have swelled, preventing release of fluid


We felt this car’s problem was probably in the brakes, because incipient wheel bearing failures are noisy, and the hub on this car was silent when driven on our lift. 

A simple “pry test” showed us that the caliper pistons moved back, and the pads were free in the caliper.  So we did not have an obvious caliper failure.  When we looked at the drip tray we saw substantial leakage from the distribution valves, suggesting wear inside.  Valves that leak externally can also fail to seal shut, venting pressure to the wheels and causing drag.  Teardown would tell the story.


Above - the distribution valves show much greater than normal leakage.  They leave six-inch puddles overnight with the cover removed.  That will be addressed after fixing the drag, unless distribution valve failure is the cause of drag.

We began at the right rear, where the failure was originally observed.




Above - the driveshaft yoke as removed by the Rolls-Royce hub tool

We began by taking apart the hub and brake assembly.  Why?  Because the hub has to come apart either way, if it’s been that hot.  At the least, we would need to repack the bearings.  At worst, we would find a failing or failed bearing.




Above - the rear hub bearings,  Note the outer is greasy, while the inner bearing is about dry.  Neither has failed, but without attention, the inner bearing would have burnt up soon.

As you can see, we found a hub that had not yet failed, but had lost all the grease from the inner bearing.  Probably a combination of age and heat.  Either way, at 40 years of age, this car will get new bearings and seals.

Now we moved onto the brake caliper.  We popped off the rubber dust covers and applied shop air to pop out the pistons.  This is usually a simple process where the pistons pop out into your hand.  Not on this car.  The small pistons (the ones that go to the backup master) were very tightly stuck.  The main pistons were tighter than they should be too.



Above - a failed caliper piston.  The rust line is where the piston enters the seal area.




Above - the other side was worse



Above - a closeup of the corresponding caliper bore shows rust damage and roughening.  This surface must be perfectly smooth and even.  If it's too rusty to clean up, we split the caliper, bore it oversize, and press in a sleeve.

Once they were out and wiped off the reason was clear – rust, coming in from the outside.  When you look at RR brake pistons you can see rust inside, or coming inward from outside.  Rust on the inside is very bad, because it means there are corrosives in the fluid and the whole system must come apart.  Sludge in the caliper means the same thing – complete teardown.  Luckily this car did not have those problems.

We used a scope to look inside the brake pipes and found them clean.  When you look inside you are looking for swelling of the rubber, or rust in the steel.  We didn’t see either on this car.  So no line repair is needed.

The brake pads tell their own story.  Look at the left and right sides, next to each other.  As you can see, the pads on the side that smoked are melted and glazed on their surfaces.  The glazing is concentrated on the part of the pad that is above the sticky small pistons.



With these observations, we can paint a picture of how and why this system failed.

  1. The small pistons in the right rear caliper corroded and became tight, which made the brakes drag slightly.
  2. When the car got up to speed the drag got the brakes hotter and hotter, which eventually warped the rotor and caused the shake.
  3. The hot rotor melted the grease causing some to run out onto the rotor as the car cooled.  That was what we saw, and it caused the smoke the driver saw.
  4. The hot brakes boiled the fluid, which caused the pedal to go to the floor, and light the lamp
  5. The leaking distribution valves were not part of this failure, but they leak excessively and should be changed before they cause a bigger problem


So what now?



Above - the other side's caliper pistons were not dragging as much, but they all show rust breaking through, which is the seed of failure whether now, next year, or two years hence.  Best fix it before it leaves the car stranded.

With the piston damage we see in back, the wise action will be to overhaul all four calipers.  The right rear pads are visibly damaged so we will change the rear pads too.  Seeing the grease run out of the inboard bearing, we should pull apart the other side hub and service it too.  Having pulled apart the hubs (an all-day job requiring a RR hydraulic hub tool) we should replace the rotors.

In the front of the car we’ll be servicing the calipers, and we’ll check the hub bearings.  We will also replace the leaking distribution valves, and make sure the valve block linkage is in good order.  Check back and we’ll report on this job as it progresses.

Bear in mind that the damage you see in these photos is on a well-maintained car.  This is not damage from neglect.  The brake fluid is clean and clear, but nothing stops rust from outside, especially in a car that lives near the ocean, as this car has for 30+ years.



Above - For comparison, here is what fluid looks like in a system that was neglected.   Needless to say, that car has much bigger problems.

It’s just another day in the life of a Rolls-Royce repairman.

No comments: