New clutch master cylinder leaking from cap - does it need rubber seal?

[Bumpa]

I fitted a new plastic reservoir clutch master cylinder to my '71 B. After a couple of journeys I found fluid on the heel mat and the cylinder was half empty. I have taken the pedal box cover off and it is NOT leaking from the push rod end of the cylinder, which is what I expected. The fluid has been getting past the cap and dripping down the outside of the reservoir. Today I tried pressing the pedal with the cap removed and I got a fountain of fluid spurting out of the top of the reservoir. Is that what you would expect to happen?

The plastic cap has no rubber seal and I guess is meant to seal down tight without one. I've never had this sort of thing happen before so I would like to know if the cylinder is behaving normally in squirting fluid upwards from the reservoir when the cap doesn't seal. I could try finding a flat rubber seal of the right size.

I dread having to get a replacement as it is a PIA to replace and bleed as most of you know. And of course it has wrecked the new paint round the pedal box.

[Vic Butler]

It could be the flexible hose has collapsed internally. Vacuum bleeding is probably the best way.

[Bumpa]

Thanks for the thought Vic, but all the components are brand new following the rebuild just finished. What I've never seen before is the fluid being ejected upwards from the reservoir. With the cap off it gets to a height of about 6 inches before splashing down and ruining my nice new paint. Just to make sure that it isn't normal behaviour, I took the cap off the master on my Dolomite and pressed the clutch pedal. The fluid didn't move at all, which is what I would have expected.

As anyone who has done clutch hydraulics on the MGB knows, fitting the master cylinder and bleeding the system is a right royal pain and now I have to do it all for a third time. On first putting it together the new slave cylinder started dripping overnight. The well known supplier quite catagorically said the component wasn't faulty, but after I sent photos of the drips and the fluid collected in a tray, they had to concede and sent a replacement. So that was bleed number two.

Now the faulty master is going back to a different supplier and when a new one comes that will be bleed number three. Although this car was roadworthy last autumn it has only managed about 100 miles because things keep failing. It is driving me daft, especially since Mrs H tells me there's loads to do in the house. Who said that retirement left them with time on their hands? I've never been busier.

[Ian F]

If I remember correctly Mike, both hydraulic reservoir caps have breather holes, so there should never be any internal pressure forcing fluid upwards. It's a strange one, never come across this before, but it must be a faulty MC I think?

Ian F

[Vic Butler]

It appears that pushing the pedal Is pressuring the master cylinder and the fluid is not going down the outlet to the clutch slave or the latter is seized. I know you have replaced everything but some new parts still can be faulty. It might be worth slackening the bleed nipple on the slave cylinder and seeing if fluid comes out when the pedal is pressed.

[Allan T]

Related to the question without in any way answering it, if you're replacing your master cylinders, hang on to your old caps.

My new cap split after a few weeks. The old one, re-fitted, continues.

Are the new ones being overtightened to near-splitting-point at the factory?

I don't know.

But cap failures seem to be a fairly widespread experience lately.

[Ian F]

Alan - exactly the same thing happened to me. The OE plastic cap on my new MC split after a week or so. Fortunately I had saved my old one which is still on there.
Moss did send me a replacement cap which also split, so rubbish quality.

Ian F

[Bumpa]

Well I have the second new master cylinder fitted and all is well. Even bleeding the system didn't take long. A half dozen pumps from my wife's danty foot and it worked fine. I had to keep jumping up from underneath of course as two pumps nearly emptied the reservoir - why are they so small?

Vic thought the fluid from the faulty cylinder wasn't going down the pipe, but it was, as well as squirting upwards at the same time. The car drove perfectly with the clutch operation being entirely normal. I only discovered there was a problem when I saw the fluid on the mat by my feet.

[Vic Butler]

Pleased you sorted it. Unfortunately new parts can still be of dubious quality these days. MG doesn't care about its heritage unlike Land Rover. I can still buy certain parts for my 1975 Land Rover from a main dealer but I certainly can't for my 1977 MGB GT.

[Ian F]

If I remember correctly Mike, both hydraulic reservoir caps have breather holes, so there should never be any internal pressure forcing fluid upwards. It's a strange one, never come across this before, but it must be a faulty MC I think?

Ian F

Just checked an original Lockheed cap and I think I was mistaken. The cap is "double skinned", and although there is a hole in the inside, there doesn't appear to be one on the outside.
A bit puzzling, because there is definitely a breather hole in the cap of my Girling master cylinder on my Series 2a Land Rover.

Ian F