Restoring The Brake Servo Unit Vacuum Cylinder
The
internal surface of the brake servo vacuum cylinder is coated with a
dry film lubricant that can wear off with age. This prevents the vacuum
piston from moving freely resulting in erratic brake action. I suffered
through several problems trying to restore the brake servo unit on my
1967 Austin Healey 3000 before locating a dry film lubricant for the
cylinder. This is a summary of my problems and the final solution.
After
rebuilding the brake servo unit using a repair kit from Moss Motors,
the brakes locked up after the first application. I removed and
disassembled the servo unit and found that the vacuum piston was not
moving freely. The pressure differential created on the piston when the
brakes are applied moved the vacuum piston forward to amplify the
braking effort. But, the piston return spring force was not sufficient
to overcome the friction between the leather seal on the piston and the
vacuum cylinder. Consequently, the vacuum piston did not retract,
locking the brakes.
The
high friction was caused by the new locking plate seal, i.e., the
rubber compression ring that holds the piston leather seal against the
cylinder. Checking with Moss Motors, I was informed that the seals
supplied with these repair kits were too large in diameter. There were
no seals of the proper diameter available, and of course I had thrown
the old seal in the trash.
I
shaved down the seal until the piston moved freely, reassembled and
installed the servo unit, and guess what? The brakes locked up! Not as
severe as before, and the brakes would release by tapping the servo
unit with a rubber hammer. However, this mode of operation was not
acceptable, since it severely bruised my ego to hold up traffic at each
stop light with the hood (bonnet) raised on this beautiful automobile,
so off with the servo once again.
Upon
disassembling the servo unit I discovered that the coating on the
vacuum piston cylinder near the retracted piston position (brakes off)
was essentially gone. The repair manuals indicated that "no lubrication
is needed for the piston seal since the cylinder is specially treated
during manufacturing". No problem, just find out what this special
treatment is, and retreat the cylinder surface. Wrong, I called
everyone that might know and ran into a blank wall. They all said, if
you find out let us know. That is why I am writing this article.
I
did find out how they treated the surface during manufacturing, but a
chemist friend of mine put me on the right track. He indicated that a
lot of progress has been made in dry film lubricants since the '60s and
recommended a local company that specializes in these coatings. I
investigated the products, stripped and recoated my vacuum boost
cylinder, and guess what? My brake servo works fine, I can stop my
Healey on a dime, and I don't hold up traffic at stop lights.
The dry film lubricant that I used is provided by:
Sandstrom Products Co.
224 Main Street, P.O.Box 547
Port Byron, IL 61275
Contact: Patrica A. Morland
Lubricant Division Manager
(309) 523-2121 ext. 1084
Toll Free (800) 255-2255
The
dry film coatings that Sandstrom Products markets have molybdenum
disulfide as the primary lubricant combined with a lacquer-like
additive for adhesion to metallic surfaces. They provide two basic
products. One is 26A, an air dry lubricant that will withstand
operating temperatures of 300ºF. The other is 9A, a heat curing
product that will withstand operating temperatures of 500º F.
I
used the 26A product since the operating temperature of the servo is
below 300º and it was much more convenient to apply. This product can
be obtained in an aerosol can for about $10/can. Try it. You will like
it.
By Richard Strunk