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MG TD TF 1500 - Master Cylinder rebuild

I was having problems with a low pedal on my TF even after bleeding the brakes so decided to rebuld the master cylinder in hopes of rectifying the problem. Removal and dis-assembly problem free. Noted the rubber cups all marked Lockheed so assumed the clinder had not been recently rebuilt ( replacement cups from Moss not marked as Lockheed. As I am far from a master mechanic here is my question?problem. Lubed all parts with brake fluid and carefuly refitted. When piston is compressed fluid flows as should but piston fails to return fully to the stop held by the spring clip. I believe sufficient care was taken when inserting the new cups but something seem to be binding, cylinder moves freely but seems to slow before fully returning. Any and all tips suggestions or appropriate prayers and incarnations to the MG gods most welcome!
Jeff Payne

There could be several problems. First, the power to return to cylinder to its resting position comes from the springs on the brake shoes. Make sure they're properly placed. The master or slave pistons could be corroded and sticking. Moss sells a lubricant just for this. There could be corrosion in the brake line which the high pressure of the master cylinder can overcome but is not overcome by the low return pressure of the slave cylinders. Finally there is an adjustable stop on the pin that compresses the master cylinder make sure it is adjusted to give about 1/2 inch of freeplay.
Russ Oakley

And would the little tiny pinhole relief opening in the MC keep anything from returning to original position...certainly kept my brakes on tight....?
gordon lawson - TD 27667

The master cylinder should not need the pressure of the brake shoe springs to return the piston to it's normal position. Additionally there is insufficient travel of the shoes to push the M/C piston back to it's rest position. The M/C piston should be pushed fully back to it's home position by the internal spring in the cylinder. If it is not doing that, then I would suspect some internal binding in the cylinder. Did you hone the cylinder before reassembling it? If not, there could be corrosion on the wall of the cylinder that is catching on the seals and preventing them from traveling all the way back. The internal wall of the cylinder tends to build up a small ridge of dirt at the rest position of the seals and with new seals, possible a small bit longer than the old ones (or longer from lack of wear), they are hanging up on the ridge.

The small relief hole should not keep the M/C piston from going all the way back, although the piston not going all the way back far enough to uncover that hole, will be the cause of the brakes not releasing as the fluid doesn't have any way to vent back into the reservoir. This does bring up other problems that might occur in the M/C. As I recall, there is check valve at the far end of the spring that may not be properly assembled or malfunctioning in some way.

If you remove the M/C to check it out, jury rig a tube from the outlet port and direct it back into the reservoir, so that the M/C can be bench bled. This also give you a chance to see if the piston is returning all the way prior to installing it in the car and hooking it up. Good luck - Dave
David DuBois

I had that problem on my last m/c rebuild, too. It helped to stuff some Girling red grease into the dry side of the cylinder and run the piston back and forth a few times. It still isn't perfect, returning a bit slowly at times, but at least it doesn't leak! Hopefully it will stop sticking before it starts leaking again. If I felt in danger, I would get another kit (from a different supplier) and try again. I think my seal was too tight, but as I said, it's more of an irritant than unsafe. You could stretch the inner spring a bit. They used to include new ones in the kits, don't see that now.

Look at the MGA section. His problem was solved by reseating the seal correctly on the piston. Hmmm.
Tom (again)

Have mentioned it before but when my little relief hole was plugged, i used a pin (and about an hour) to get it opened up... Saw a set of MIG welding tip cleaners later on and one of these would have been perfect...basically a pin sized file.
gordon lawson - TD 27667

When I rebuilt my Master Cylinder I had a stainless steel sleeve put in which returned the bore back to standard and I had the same done recently on my B's clutch slave cylinder a reletively inexpensive upgrade which improves the cylinders a lot.
It might be worth a try
Wayne Styles1

Who installed the SS sleeve for you and did they press it in or did they machine threads on the sleeve and the cylinder?

John Progess

I just sent mine to White Post who does the whole rebuild. They bond the sleeve in no fancy threading. Lifetime warrantee to the purchaser.
D Clark

Gents, as always thanks for the input. I honed the cylinder ( noticed some sludgelike build up in spots) the honing seemed to do the trick, piston now returns fully to the stop....hopefuly this will solve low pedal when re installed
jeff payne

Check that little hole...its down in the bottom of the rearward large hole..... only the tip of a pin will go through... a whole small needle will.
gordon lawson - TD 27667

This thread was discussed between 05/01/2006 and 07/01/2006

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