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MG MGA - MGA master cylinder rebuild

I've just finished rebuilding the brake/clutch master cylinder for my '57 roadster, and I'm having trouble getting the pistons to come back to their 'rest' positions after pushing them into the bore. I've kept them wetted with brake fluid, and I've done the 'force them in and blow them back out with air' trick to the point that they will usually come back to rest if I push them all the way in, but won't return if I only push them part way in. I've cycled them full-stroke probably a hundred times, and I still have the problem.

Is it possible that the springs in the cylinder bores have weakened since 1957? Should I order new ones? Any othe ideas other then 'keep cycling them'?

Thanks

Howard
Howard Battan

Howard, It is possible however very unlikely. If the springs are at fault, then most likely they have rusted such that the wire diameter has been significantly reduced. This would have been noticed when doing your rebuild.

Most likely the cause is either a) the rubber seals did not get properly set into the grooves on the piston or 2) the seals are incorrectly built. Check the archives. This has been a problem in the past. I have to admit, I have run into the bad replacement parts scenario on this issue.

Chuck
Chuck Schaefer

Thanks, Chuck. The springs looked fine - no rusting noticed.

I've been in contact with Kelvin Dodd at Moss Motors, and he sent me the info on the past problems, which were apparently due to improper pistons being used with the normal seals. Mine fit the discription and dimensions of the normal pistons. I'll keep working on it.

Howard
Howard Battan

I stretched my springs, figured they were old and collapsed,seemed to fix the sticking. Once in use they should free up mine did. What fluid did you use ? silicon is not as slippery and makes this problem worse but it is all I will use.
R J Brown

Howard,
Spend some time reseating the seals on the pistons. They may appear to be seated, but they probably are not. All it takes is a tiny bit of mis-seating to cause the seals to bind in the bores. Work the OD of the seals against a smooth surface. Press them down, either into the groove or onto the flange. When you put the piston into the bore, lubricated with some brake fluid, it should pop back out fairly eaisily. I have recent "experience" with this! Went so far as to buy a new set of seals. Also, there are two styles of pistons. One has a "flange" that the seal fits over. The other has two flanges and the seal fits between them. Much easier to get the seal seated properly on the second type. I was lucky enough to have some of each. In the "old" days, when you bought a rebuild kit, the pistons were included with the seals already installed.
Good Luck,
Gerry
G T Foster

I had this problem the last time I rebuilt a master. I took the boot off and stuffed some more of the special grease into the cylinder. That made it much better. I do think I have a swollen brake hose, though. That seems to keep it from returning well. Maybe the seal I got was a bit too big, too. At least it doesn't leak, a year later.
Tom

I had the same problem, and by just using a new outer seal and retaining the old inner seal, it went away. The problem is with the new Moss seals, they are just wrong.

Try getting a kit that is genuine Lockheed, it will fit immediately, and then the problem is gone for good.
dominic clancy

I agree with Gerry. I took one step further by cleaning the piston groove, making sure there was no residue from the old seals. Plus I took crocus(spl) cloth (used extra fine #1) to remove residue from the piston. My .02 cents.. Mike
mike

RJ, I used DOT 4 LMA fluid from NAPA. And I've ordered new springs, just in case.

Gerry and Mike, I'll double check that the seals are well seated when I get into it again. I did thoroughly clean the piston with a wire wheel to remove all signs of old seals, dirt, etc. It FELT like they were well seated at the time.

Domenic, the kit and seals I got from Moss were the Lockheed parts. Not only the box, but the rubber parts had 'Lockheed' on them.

Tom, what special grease do you mean? I just used the DOT 4 brake fluid to lubricate the seals and the bore.

Thanks to all for the helpful hints.

Howard
Howard Battan

This thread was discussed between 06/10/2005 and 07/10/2005

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This thread is from the archive. The Live MG MGA BBS is active now.