Brake Servo

Restoring The Brake Servo Unit Vacuum Cylinder By Richard Strunk

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 300o F. 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.