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MG Brakes

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Alfred Corbin

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I'm working on a 1953 MG TD roadster. My question really is not specific to MG's, but maybe someone out there can help.

In the master brake cylinder there is a valve at the output end, close to where the fluid leaves the MC for distribution. It looks like it may be a check valve. What is the purpose of this valve? If it is indeed a check valve, that would tend to permanently lock up your brakes after an application of pedal force. I'd appreciate any information I can get.
 
Hi Alfred,
My MGA has the same type of check valve as the MGTD. What the service manual 'officially' calls it, is a Combination Inlet and Outlet Check Valve. That certainly clears up any and all questions that anyone might have. All kidding aside, what this combination check valve consists of is a valve body, valve cup (which sits inside the valve body), and a valve washer (which is a large rubber washer that the valve body presses against). When the brakes are applied, the piston forces fluid through the holes in the side of the valve body, past the valve cup and on out to your wheel cylinders. This overcomes the brake spring tension on the brake shoes, and of course your car stops. When you release the brakes, the spring in the master cylinder pushes the piston back and also has spring pressure on the valve body. However, the spring tension in all of your brake shoes forces the wheel cylinders back to their closed position. This causes fluid to be forced back into your master cylinder, however the valve cup is forced closed, causing the fluid to flow past the valve washer and around the outside of the valve body. If you look at your valve body, you'll note that the outside is hexagon shaped. This allows fluid to run around the outside of the valve body. Of course the spring pressure inside the master cylinder will finally force the valve body tight against the valve washer. This maintains a small pressure (approximately 8 pounds per square inch) in all your brake lines and wheel cylinders. The service manual says that this would 'ensure that the cups of the wheel cylinders are kept expanded'. It also aids in bleeding the brakes by preventing air from back flushing to the master cylinder. I am sure this is as clear as mud, which hopefully is not the color of your brake fluid!
 
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