100-4 Master Cylinder

By Roger Moment

Originally published in the Austin-Healey Magazine, November 1983


Austin Healey 100/4 brake master cylinders can develop a condition whereby they don’t allow bleed-back of brake fluid from wheel cylinders into the reservoir. Symptoms of this are a very HIGH and HARD brake pedal, which results when the brake shoes don’t retract upon release of pressure. Consequences are dragging brakes that cause the drums to overheat, the main concern, and results in excessive lining wear. Repair is easy, involving replacement of the large diameter seal, but to understand why it is worth delving into how the master cylinder work’s.


Page M/3 of the 100/4-shop manual shows an exploded view of the master cylinder components. This is reproduced in the accompanying figure along with a crossectional view of how things look assembled. When you put your foot on the brake, the piston moves forward causing the bleed-back hole to enter the high pressure chamber and sealing-off the reservoir from any back-flow. Upon brake release, this hole lines up with the shim which has raised areas on both sides to allow fluid to pass between it and either the rear surface of seal #1 or the internal housing bulkhead, which contains holes leading back to the chamber fed from the reservoir. If there is no gap between the shim and the seal or bulkhead, the fluid can’t bleed back and the brake pistons remain extended. Subsequent brake application find no free travel to the brake pedal, and as the brake shoes are essentially always contacting the drums, you are in trouble.


This cause of a no-gap condition can be two fold. First and most likely, the seal #1 can get soft and swell, filling the gap between it and the shim. Installing a new seal will create a proper gap and solve the problem. A second cause, and one that would show up with a new or just-rebuilt master cylinder, could be that the raised portions of the shim are not pronounced enough to allow adequate fluid flow through the gap. I had a “defective” shim like this and replacement with a more textured one solved my problem.


Finally, working on brakes and experiencing a swollen seal serves to make one appreciate even more the advantages of silicone brake fluid. First, when disassembling the brake system fluid spillage onto the frame is nigh-on impossible to avoid, and, with Girling fluid, paint damage will result. With silicone fluid there is no paint reaction and you only need to wipe the frame with a rag when you are done. In addition, silicone fluid, being inert, should not react with the rubber seals to cause problems in the first place. Another reason why the cost spent on silicone brake fluid up front can pay off in the long run.


I would like to acknowledge comments from Ken Walsh in analyzing this problem and figuring out the solution.


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