K16 – Master Cylinder Return Springs

previously published as “Rebuilding Rootes’ Master Cylinders” by Tiger Tom in the January 2003 RootesReview and updated in the November 2004 issue

In rebuilding your Sunbeam’s clutch or brake master cylinder, it is important to inspect the long, internal return spring for wear where it rubs the cylinder bore during use. Over time, the spring coil edges can be worn flat and shiny. In rare cases the wear is so severe that the spring breaks.

As a result of this wear, the designed spring rate becomes reduced, preventing the piston and seal from completely returning to the parked position when the brake or clutch pedal is released. If the piston and seal do not return completely, the fluid from the reservoir cannot replenish the quantity that was just displaced. Symptomatically, the next time the brake/clutch pedal is depressed it will likely go to the floor, with nothing happening. It will feel as if there is air in the line, when in fact there may be none. In rare cases, a residual hydraulic pressure may remain.

Typical brake problems you are likely to experience as a result of a worn spring are calipers “hanging-up” – sticking or failing to release. If the factory servo is installed, you may think that it is sticking.

Problems with the clutch hydraulics can include incomplete disengagement, or failure to disengage at all. When experiencing either brake or clutch symptoms, the driver will typically “pump the heck” out of the pedal. This rapid action will sometimes aid full return of the piston and provide temporary relief.

The effects of a weak spring are especially acute immediately after a cylinder is rebuilt. This is the time when the seal is at its tightest and friction forces are their highest. The spring wear problem is exacerbated by silicone fluid. Silicone has a lower lubrication coefficient than does our beloved Castrol fluid.

The high spring rate of a new or unworn spring is required to overcome the greater forces when using new seals with Silicone fluid. I have noticed that springs with flat spots worn along them can fail to return the piston and seal to the parked position. Note that two types of springs are found in our master cylinders. The offending one is a linear compression spring about 3-3/4 inches long and about the same diameter as the bore, minus a couple thousandths of an inch. The other spring, which rarely poses a problem, is much smaller than the bore diameter and is attached to the piston by a small rod.

Always check the condition of the spring while rebuilding the cylinder. Replace a suspect spring with a good used or new spring.