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MG MGA - Master Cylinder Woes

I am just about at my wits end! I rebuilt the master cylinder last week and have since had it out of the car two more times due to one or both pistons not returning when the clutch/brake pedals are released. I have honed both cylinders and still have the problem. The pistons are of the original style, so I don't think the problem is with the Moss seals (mentioned on Barney's website and in the archives). His site was extremely helpful on the rebuild with all the good pictures. I am able to get the pistons out by using a screwdriver in pushrod hole and pulling the pistons out. I am thinking that I will have to have the M/C re-seleaved.

Any thoughts or suggestions will really be appreciated as always.

Hope you all had a Merry Christmas.

Don Carlberg
1960 MGA 1600
D. R. Carlberg

Before installing the unit, lubricate with plenty of brake fluid. Press in manually. Return with air pressure. Repeat as many time as necessary to get it to loosen up, possibly 20 cycles or more. Don't give up. Give it 100 cycles if necessary. Always works for me.
Barney Gaylord

Barney;

please clarify for me the term "RETURN WITH AIR PRESSURE'' i'm not sure i understand exactly what you mean.
tks,
garry irwin
gei irwin 1

Push the piston in. If it doesn't return on its own, apply air pressure to the outlet port to push the piston back.
Barney Gaylord

If already installed and full of fluid light air pressure (turn the regulator to 10 or less) held on the fill hole (I used a rubber tipped air nozzle large enough to seal on the fill hole) with an assistant pumping brakes will free it up after as many cycles as it takes. DON'T do this with other than silicon as you'll probably make a mess. This is how I got my 58 master to work after fighting it.
Doing it off the car in a vise blowing back through the outlet with just enough fluid to lube it would be less messy.
I just didn't want to fight reinstalling the lines and rebleading the system by taking it off.
R J Brown

Thanks for all the ideas. I am getting pretty good at removing the M/C at this point. Neither the Clutch or Brake line brackets have been reinstalled, so it is much easier to get it out at this point. I'll update my progress a little later this week.
Don Carlberg
D. R. Carlberg

D.R.,
I recommend that you very carefully re-check the installation of the main seals on the pistons. Depending on the style of your piston, it is very difficult to get the seals seated properly. They must be completely seated in the groove. Often, although they appear to be seated, they are not. The smallest amount of mis-seating will cause the piston to not return. Work the seals into the grooves using whatever break fluid you're planning to use as a lubricant. Use a modifed (blunt nose) right-angle spring hook to help seat them. I suspect that this will cure your problem.

Good luck,
GTF
G T Foster

I had exactly the same problem when I did my master cylinder earlier this year - both pistons stuck in. I managed to blow them out with a foot pump held against the other end - put a cloth over the piston as it came out at a rate of knots eventually - just missed my face and bounced of the wall behind me. I then put plenty of fluid around the seal after soaking it for a couple of hours and putting loads in the bore and proceeded to work the piston in and out (as described earler by another writer) using a screwdriver and tapping the end until it popped out again. I had to do this a dozen or so times. It now works perfectly. If you are doing the brake one - don't forget the non return valve - mine had the valve in the clutch and not the brake when I first dismantled mine - no wonder the fluid had drained out of the brakes with rather startling effect when I drove it. Cheers Cam (secretary West Lancs MG Owners - www.westlancsmg.co.uk)
cam cunningham

Just a quick update. I compared the piston seals per G.T.'s post. There were remarkable differences in the way theylooked on a side view. The seal on the piston that works freely in both cylinders had an OD that was less than the piston on the part of the seal that sits in the groove. The flanged part of the seal had a larger OD. The seal on the problem piston had a larger OD over part of the goove. It was as though the goove was smaller than the part of the seal that fits into it. Anyway, I have ordered another rebuild kit and will hope to get a proper fit on the next go-round.

On the bright side, my master cylinder removal time has dropped by at least 50%! I have some other projects going on as well (installation of the spin-on oil filter adapter, routing my oil cooler lines through the bulkead below the heater fresh air intake, and installing the short heater air intake hose), so this little M/C upset is no big deal.

Thanks again for all the good suggestions and assistance. Have a safe and happy new year.
Don Carlberg
D. R. Carlberg

Sometimes it is better to walk away from a problem for a few minutes. After finishing my last post, I went back to the M/C problem piston seal. I was finally able to get it properly seated in the groove by gently trying to pull the seal over the end of the piston. Like magic, the seal profile began to look correct as I worked the seal around the piston end. Reinstalled it and worked it back and forth a couple of times and it worked feely in the M/C.

Thanks again to all of you!

Don
D. R. Carlberg

Don - glad to hear it has worked out for you! Then again, I'm not surprised as I know you've been driving that particular MGA since the 70's, and have torn apart and rebuilt just about every part in it!
- Ken
Ken Doris

Lessons Learned:

The saga continues! It took awhile, but I got the clutch side bled without any problems. Got nothing from either rear brake bleed screw, so I started with the fronts and got them bled rather easily. I went back to the rears and started checking the connections to see if I could determine the location of the blockage. I pulled the right rear drum and found no evidence of brake fluid in the brake cylinder. I had good flow to the inlet side of the rear brake hose but nothing coming out the other end. I have removed the rear brake hose and it is plugged. Could not even blow air through it. I guess I have been braking with just the front disc brakes for some time. Have ordered a new hose and rear brake lines....
Thanks again for all the assistance.
D. R. Carlberg

Don - you say bleeding the clutch was no problem. Did you do the trick of removing and inverting the slave cylinder or was it just by "standard" methods?
- Ken
Ken Doris

Ken - Vicki got a great workout pumping the clutch pedal. I had the bleed nipple out for the first part of the bleeding process. It took an eternity, but eventually the pedal firmed up. I went back and found that I had way too much clearance on the clutch pushrod so the effective piston stroke was reduced which I am sure was a big contributor to the amount of time and effort it took to bleed the slave cylinder. I looked all over town for an e-z bleed but could not find one.

I am a little concerned about the LR brake wheel cylinder. The banjo bolt head is pretty torn up - it will be a real pain to get it loose if I end up replacing the wheel cylinder. I was able to loosen the RR. On the bright side - the rear brake shoes are practically new. I replaced the rear brake drums in the mid 90's and they look good as well.

I'll get to rest for a few days waiting on the next round of parts to come.

Don
D. R. Carlberg

This thread was discussed between 25/12/2005 and 06/01/2006

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