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MG MGB Technical - Disc Brakes Dragging

I have a MG TD with MGB front disc brakes and MGA rear drum brakes operated by a MD TD master cylinder. My problem is that 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 wheels, I am able to rotate the wheels but they drag more than the light kissing that is normal. Additionally, the disc are very hot and will easily burn my finger tips when touched. I have verified that there is sufficient clearance between the brake pedal and the push rod on the master cylinder (e.g sufficient pedal free-play) The pedal seems to be a little spongy indicating air in the system. Could this be the cause of the dragging brakes? Any other ideas on what may cause this problem.
Frank Grimaldi

Frank. How old are the two front brake lines? They can have an internal breakdown not seen externally.
Also have you checked the smaller of the two holes in the master cylinder. sometimes with a new rebuild kit the cup slightly covers the smaller of the two and inhibits the return of the fluid to the resevoire. Try just popping the bleeder valve and see if this helps the release of the pads.
If this works you will have to cut about .010 from the face of the piston. This should allow the cup to clear the small return orofice.
conrad sanders


More info...

1. One of the calipers is a new remanufactured unit and the other was recently rebuilt using new seals and pistons.

2. The brake fluid is the Silicone type.

3. The flexible rubber brake lines and the steel brake lines are old, perhaps 12 to 15 years old, but the car has not been driven for the past 10 years and was stored indoors in Southern California.

Frank Grimaldi


Your problem identification and possible solution seems quite plausible. Please be more specific with respect to the two bleed holes.
1. Where are these holes located?
2. Are they potentially blocked by the piston or the rubber seals?
3. When you say remove about .010" from the face of the cup, I presume that you mean grinding the flat portion of the piston that is inside the caliper and the side in contact with the fluid. Is this correct?
4. The problem with the brake drag occurred with both new and worn pads. Would the blockage you described occurred with new and worn pads?

If I have more questions, may I call you? Please include your phone number with your response or e-mail me at
Frank Grimaldi

DOT5 silicone brake fluid has low lubricity, sometimes allowing the pistons of calipers and/or brake slave cylinders to bind in their bores. Despite what some may tell you, it is not intended for use in automotive brake systems and a warning should so state on the container that it came in.
Steve S.

The two holes are located inside the master cylinder. They are visible through the fill cap. The rearmost one is very tiny I use a #60 drill to clean it out. Also if you have welding tip cleaners one of the tiny ones should work. The rubber cup if it is too thick will block the rearmost hole. Ten you have to remove .010 approx from the face of the piston itself . This allows the cup to rest slightly further aft in the cylinder.. The brake hoses are too old and I would not trust them. you can check them for blockage by opening one of the bleeder valves slightly and then the disc should be a lot easier to turn. Do this on both sides.
Actually I suspect the hole in the master is blocked by the cup or dirt. It can be a bit of a challenge to clean out.

Do not force anything through that hole while the pedal is full back as if the cup is blocking the port it could be ruined. Just press and hold the pedal down and then try cleaning the hole. If it's clean then with the pedal full back you can feel wether it's a coverd hole or not. You may have to bleed some fluid out as you depress the pedal to get the piston to clear the hole. This means that the wasted portion is actually under the ports at that time.
It is not a real problem to remove the master. Actually it can be removed from outside the car just by reaching under and with a 3/16 W or 7/16 wrench undo the brake line to the T fitting. Then just remove the two bolts and pull the complete master out. This will leave the plunger attached to the pedal.
conrad sanders

Frank If you are still running the TD master cylinder the disc brakes will drag. The TD master has a line pressure valve that holds about 10lb pressure in the lines all the time as most drum brake vehicles have, enough to hold on the disc brakes. Drum brakes have return springs strong enough to overcome the valve pressure. You should be able to remove the line pressure valve. It will make the brakes a bit softer but should fix your problem. Denis

The interior of the flexible rubber brake lines will fall apart over time and if a portion breaks loose it may create a flap that can act like a one-way valve restricting the flow of fluid. I noticed that my front left wheel on my 74 MBG was dragging. The drag disappeared after changing the rubber lines.
R Volkwein

I know a lot of people are reporting brake hoses that close up as they get old and I am not denying the fact that it can happen, but people are missing the point in this situation. Frank is running disc brakes with a drum brake master cylinder. Almost all drum brake M/cyl have a valve in the end to hold a small amount of residual pressure in the lines. "A disc brake m/cyl does not. This valve MUST be removed if fitting disc brakes or they will drag. Maybe frank has other troubles as well, but this problem must be addressed first. Denis

Were all the rubber parts of the brake system replaced when it was changed to DOT 5 fluid? Rubber that has been used with normal brake fluid and then changed to silicone may swell, sometimes a lot. I have been using silicone fluid in both an MGA with drum brakes and an MGC with disk/drums for 16 years with absolutly no problems with the brake system, but I did replace all the rubber parts and cleaned the lines before changing over. If the check valve mentioned by Denis is in your master cylinder it could certianly cause yor problem.
John H

Regarding DENIS4, Victoria, Australia comments, I presume that the line pressure valve is the small rubber cup that fits into the valve body at the end of the return spring. Is this the valve that should be removed? Should I also remove the rubber valve washer that mates with and seals the rubber cup? Please see Pages M4 and M5 in the MG midget Series TD and TF - Workshop Manual.
Frank Grimaldi

It does not matter about the rubber washer, not doing anything once the valve is removed. Denis

This thread was discussed between 01/09/2008 and 07/09/2008

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