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MG TD TF 1500 - Sticking master cylinder

After discovering that even with all new wheel cylinders that I still had no brake pedal pressure, I got a rebuild kit from Moss. The bore was excellent and after some slight honing just for good luck, I assembled the parts in the MC after a good cleaning. When I depressed the piston with the rod on the bench it wouldn't return to the original position. I worked the unit many times until the piston would come back to the starting position smoothly and installed the rebuilt master cylinder. It acts the same now with a sticking piston as it did on the bench prior to freeing it up. It seems as if the internal spring just doesn't have enough pressure to push the piston back to the starting point easily. I can actually hear the rubber parts "squeak" in the bore of the cylinder when it sometimes moves back and forth to the proper position. It will go back to the original position but only after about 5 minutes without use.
Could it be that piston rubbers are too large in diameter and not allowing the piston to move freely? I had every thing well lubed up before assembly. If this is the case, what does a person do to solve this problem. If it is not the problem, does anyone have any ideas?
This is not a high tech job but I am missing something. Never have had so much grief with brakes on other cars as I have had on this TD. Fortunately I have used silicone fluid or I wouldn't have any paint left on the car!

Mark
Mark Strang

I took the master cylinder off and disassembled it. It appears as if the secondary cup is hanging up in the bore and won't slide back and forth. The bore of the cylinder mics out to about 0.880" while the piston is at about 0.874". The part of the secondary cup that fits over the piston lip, and holds it in place, is at about 0.895. The cup working diameter is about 0.925. It seems to me that the part of the cup that fits over the lip in the center part of the bore is oversized since it is an interference fit with the bore of the cylinder and has no way of compressing during the sliding motion. Looks like it is at least 0.015" too big. The working diameter of the cup will compress as it moves back and forth in the cylinder as it should but I don't see how the first part of the that cup can slide freely. There are some signs of wear on that portion which would verify my theory. It would seem as if the only part of the cup that should contact the bore is the maximum diameter of the cup that is moving the fluid. Does this make sense? Could I just try to polish off a bit of rubber on the offending part of the cup so that it is no larger that the piston and still be okay?
If anyone has had this happen, I'd appreciate hearing about it and how you solved the problem. The parts are not a big ticket item so it would be interesting to try this is it seems logical.

Mark
Mark Strang

The "secondary" cup must fit into the grove properly. It may appear that it is seated, but often it is not actually all the way in. Get in there, carefully, with a very small flathead screwdriver and make sure the grooves on the piston and cup are aligned. Then work the rubber back and forth, around, etc to ensure it's really in there.

I have also run into not only varying types of rubber in the rebuild kits, but different pistons as well. It may be that the rubber you got was a bit thick, and the groove in the piston is not seep enough and/or wide enough to fit it properly. I've had to modify pistons before just to fit the modern rubber bits. Compare the old cup with the new one and you will probably side a difference. But still, you should be able to work it into the groove and make it work. After that it may be a bit sluggish at first and then smooth out as the bore and rubber wear together. Hopefully you didn't hone too aggressively. The bore should be relatively smooth.
Steve S

Mark, I've resurrected a thread from last fall when I ran into the problem. I eventually got the supplier to send me a cup that fit. Good luck. Bud
Bud Krueger

Mark, I don't know if anyone has brought it up but the Moss has a link on their master cylinder section that discusses I think 4 changes for the plunger through the years and the proper kit.
I am finally hooking my brake system back up and plan to send my master cylinder to White Brothers for brass insert, rebuild and lifetime warranty. It might be a little more but shouldn't have to worry about it again.
R Taylor

Thanks for the comments. I guess that I was not too far off on my thoughts. I'll try the sugggestions and see what happens.

Thanks,

Mark
Mark Strang

Steve, Bud and I went through this before. The new cups do not fit original pistons, and/or those made through at least 1989 or so (maybe later?) I made a detailed impression/cast of an original Lockheed and the new replacement cup, as well as measured the rubber thickness. Problems: The circumferential ridge on the original pistons is beveled/tapered toward the middle, as is the matching area on the original Lockheed cup. The inside of the replacement cup has a 90 degree sharp edge. That prevents the cup from seating, as it rides up on the bevel. The thickness of the rubber is also greater from the inside of the groove to the outside (ie the groove in the rubber cup is not deep enough). So it seems they are still supplying an incorrect cup. Yes you could turn the beveled/fillet down and reduce the diameter of the ridge on the piston. Bud and I collectively had several "original" pistons, all were exactly the same. But the correct rubber parts would be much better. Or send it to White Post as R suggested above- I did that and it came back with a nice new brass piston and a "lifetime" guarantee. George
George Butz

I think that I solved the problem due to the comments that everyone made and the old posting. I simply set up the piston with the cup in place in the lathe and "polished" the rubber part of the cup that was standing proud of the piston. I took it down with some 220 grit paper so that it was about 5 mils less than the piston. This gave me a running clearance of about 10 mils less than the bore. The working part of the cup was not touched. After cleaning all of the parts I put the MC back together and the piston returns to the end of the cylinder smoothly like it should.

This was sure a "sticky" problem. Thanks for your help. Now I just have to get it mounted without any leaks and have my patient and loving wife help me bleed the system again.

Thanks,

Mark
Mark Strang

For the record, I too had the same problem with the Moss kit! Finally gave up and bought a brand new mc because they had a sale going on. Too critical a part for me to dink with any more. New one worked perfectly ever since.
Ed
efh Haskell

This thread was discussed on 25/09/2012

MG TD TF 1500 index

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