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MG MGA - Master Cylinder Leak & Assembly

I have a recurring problem of hydraulic fluid loss from the MC. Previously I have checked all the lines, brake cylinders, clutch slave cylinder etc. I eventually discovered that fluid was leaking from the MC onto the cloth covered cabling, hence disguising the leak somewhat. I tightened the banjos and front plate and the leaking appeared to stop, or at least slow down. However, today I noticed the fluid level was well down again, so it was time to pull the MC. attached is what I found inside the brake cylinder. Can someone tell me the consequence of this please. I would add that the brakes have been working fine.

Steve


Steve Gyles

I did wonder if the cup had pulled out of its holder during the dismantling. However, it is permanently out of shape (as shown in attached image, suggesting that it has been out of position since I did an MC rebuild 2.5 years ago!

Steve


Steve Gyles

This is the slow return valve for the brakes that allows you to "pump up" the brakes if the pedal is low. The rubber cup should be completely nested inside the metal cup.
Barney Gaylord

Barney

Yes, I understand that. Without it in place would it cause a surge of returning fluid? I have noticed fluid around the metal cap and was wondering if a 'spray' of returning fluid was finding its way past the cap.

Steve
Steve Gyles

Hi Steve, When my MC leaked a couple of years ago, I found that the brake non return rubber was in the clutch instead - could have been that making it leak - but didn't seem to affect the clutch operation strangely enough. - cheers Cam
Cam Cunningham

Cam

Thanks for your thoughts. This time yesterday I would have said that I rebuilt my MC correctly 2.5 years ago (you may remember I had a blow out through the clutch banjo and lost the clutch and brakes in very short order and nearly pranged the car). Now that I have seen the detached cup washer I have become suspicious and will check everything out most thoroughly.

Steve
Steve Gyles

Any condition or location of the slow-return valve should have no affect on external leaks.

Slow-return valve omitted means you can't pump up the brakes. That is usually not a problem if the brakes are properly adjusted and have no air in the system. But it could be a problem if you encounter any condition that calls for pumping up, like for instance a leak that makes low fluid level and captures a little air in the lines. As such, this slow-return valve is a backup safety feature and should not be omitted.

Putting the slow return valve in the clutch side (where it doesn't belong) should cause slow return of the clutch fluid, thereby causing momentary clutch slippage during quick power shifts with spirited driving. This configuration should be avoided. Remember that the clutch pipe is larger bore than the brake pipe specifically to allow fast return an quick positive engagement of the clutch.
Barney Gaylord

Steve
Check the pipes at the points where they are held to the chassis with the metal tabs. I found a pinhole at one of these that was causing similar symptoms.

It was very hard to spot, and was caused by vibration against the tab wearing a hole in the pipe. All are now further protected at those points
dominic clancy

Have I finally cracked the leak? I am not sure whether to hang my head in shame for poor assembly, or whether the (new) rubber seals and cups dislodged themselves after fitting and during use. I suspect the latter - honestly. I do know how it all fits together.

I have just looked at the clutch MC piston. Picture attached of what it looked like when I withdrew it from the piston! Would this have caused leakage through the front plate?

Steve


Steve Gyles

I meant when I withdrew the piston from the cylinder - oh for the edit facility of your own posts webmaster.

Steve
Steve Gyles

Yeah, that looks like the culprit. You must have been distracted during the assembly process! The seal is very tight to fit and no way could move without stretching over the rib. In the position shown it could work (obviously did) but would not have the security and supporting shoulder so I am not surprised it leaked (from the inside - piston side of the seal). I'm sure you know all that, but thanks for sharing the experience!

Good news is, with a couple of new seals, your master cylinder is as good as new!
Neil McGurk

This thread was discussed between 12/01/2009 and 13/01/2009

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