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MG MGA - Master Cylinder Pistons

I'm in the final stages of restoration and attempting to rebuild the master cylinder. I have the rebuild kit from Moss, but have a problem with the fit of the rubber parts that fit over the end of the pistons. My problem is that I apparently have pistons that are not original equipment. Barney explains it on his web site. Check here: http://mgaguru.com/mgtech/faulty/ft021.htm

I've been unable to locate a source for new pistons, so it looks like my best alternative is to get some original pistons. Does anyone have a couple of pistons in decent shape that they'd be willing to part with?

Larry
LP Pittman

Larry, no problem I have some, if you can't get any your side of the pond drop me an email and I'll pop a couple in the post.

Neil
Neil McGurk

Neil,
Maybe I jumped the gun. I wrote the original note from work when I couldn't find anything on line.
However, I just discovered that Scarborough Faire lists them used in their catalog. If I don't have any luck with them, I'll let you know.

Thanks,
Larry
LP Pittman

Hi Larry. You might want to measure the diameter of your old pistons and compare them with the diameter of the new pistons that you intend to purchase. They may not be the same, and consequently, might not fit. Cheers, Glenn
Glenn

Glenn,
I have the new (used) pistons installed already. I didn't measure them, but they appeared to be the same diameter. The new ones are steel while the original ones were aluminum - I guess that was a common thing back in earlier restorations over the years.
However, now that I have them installed I have a problem. The pistons don't return on their own. They're stuck in the bore. I used brake fluid as a lubricant on both the bore and the rubber. I can only get them to return by using air pressure at the opposite end. I've been looking over the archives and I see two recurring themes:
1) Use new springs. I currently still have the original ones.
2) Apparently there were problems with the rubber that fits on the piston having an ID that was too small. When installed on the piston, it causes the OD to then "swell" and get stuck in the bore. I've used a Moss rebuild kit that is around a year old. When I ordered the replacement pistons from SF, I also got another rebuild kit, so I'll probably try some parts from there to see if there's any difference.

If anyone else has some other ideas, let me know.

Thanks,
Larry
LP Pittman

Lp, if you thread the right size bolt into the piston you will find there is enough purchase to move the pistons gently to and throw. After a little while the return spring will take over and push the piston back
J H Cole

This is a common problem. Continue to depress the piston and return it with air pressure. This may require a few dozen cycles (maybe more), but it should eventually loosen up and return with spring force only. It has always worked for me.
Barney Gaylord

Mine took about a year of regular use to fully break in. At least it doesn't leak.
Tom

They're moving OK now. The bolt in the end of the pistons did the trick. It did take several cycles, more so on the clutch side than on the brake side. But now they're returning on their own.

Thanks very much for the help. One more hurdle out of the way in this restoration saga.

Larry
LP Pittman

This thread was discussed between 07/06/2007 and 12/06/2007

MG MGA index

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