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MG MGB Technical - Front Disc Brake Dragging Problem Solved-Thank You

Front Disc Brake Dragging Problem Solved

My front brake/caliper-dragging problem has been solved thanks to DENIS4, Victoria, Australia, R Volkwein, Hawaii, USA and others. The solutions provided may prove useful to others that have made or are considering, converting MG TD front drum brakes to MGB disc brakes.

The original problem was described as follows:

“I have a MG TD with MGB front disc brakes and MGA rear drum brakes operated by a MG TD master cylinder. My problem is that, after removing my foot from the brake pedal, the disc brakes do not release completely thus causing the pads to drag on the disc. After returning from a drive and jacking up the front end, I am able to rotate the wheels but they dragged a lot more than the light kissing that is normal. Additionally, the disc gets very hot and will easily burn my fingertips when touched. I have verified that there is sufficient pedal free-play. The flexible brake lines are old and the system uses silicone brake fluid.”

After implementing the various suggestions, I verified that the primary cause of the problem was that a master cylinder designed for drum brakes (MG TD) was used with disc brakes (MGB). Drum brake master cylinders are designed to retain a residual pressure of approx. 5-PSI to 10-PSI in the braking system, whereas master cylinders for disc brakes do not retain any pressure. The retained hydraulic pressure resulted in a significant amount of force being applied to the caliper pistons, hence the drag and subsequent heat. The solution was to simply remove the rubber cup that is inserted into the metal cup attached to the end of the spring (see page M.5 in the TD/TF workshop manual – see part “Valve Cap”). Additionally, I replaced all three flexible brake lines. Checking the old ones I did discover that one was partially blocked but, in itself, was not the cause of the problem. I also replaced the silicone brake fluid with DOT 3,4 Synthetic brake fluid. WALLA!!! – Problem solved.

A couple of additional notes….
1. In order to ensure total pressure relief in the master cylinder and the braking system as a whole, be sure that the primary rubber seal (aka Master Cup) does not block the smaller of the two transfer holes at the bottom of the reservoir chamber. Also, ensure that these holes are not blocked with dirt. Passing a thin wire through the smaller hole and into the pressure chamber is the ONLY way to check that there is no blockage and there will be a free passage of brake fluid. Blowing through the outlet into the pressure chamber is not a reliable test.
2. If, after properly bleeding the brakes, you find that the brake pedal must be depressed more than an inch or so before sufficient pressure is achieved in the braking system, first check to see that you do not have excessive pedal free-play. If the free-play is within specification the problem may be that the steel piston and the primary seal in the master cylinder is too far behind the small transfer hole. This being the case, thin washers that act as shims, can be placed between the piston and the stop washer. Be sure to follow the information in # 1 above.
Frank Grimaldi

This thread was discussed on 22/09/2008

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