Leaking cap on the clutch reservoir

[AndyC]

Hi, I have fitted some months ago a new brake and clutch reservoir. However the clutch reservoir is constantly leaking through the cap. Is the solution simply a new screw cap or something different? I have tried a thin washer inside and PTFE tape to no avail. A simple journey of 80 to 100 miles reduces the level from max to min. The cap seems to go on tight but overtight and it slips the thread. It's ruining the paintwork!

[Not_Anumber]

Is it the thread on the reservoir that is damaged or just the cap ? Assuming the brake and clutch reservoir caps are interchangeable (those on my 1979 BGT are but dont know about others try the brake fluid cap on the clutch reservoir to see if that fits soundly and resolves the slip and you have found the guilty party

I have found a useable cap (internal diameter approx 25.3 mm) still on an old clutch cylinder here if you need one.

[Bumpa]

I can't imagine how you could lose so much fluid through the cap unless the car is regularly upside down! If the cylinder is full to the brim there might the possibility of a little seepage round the cap with the movement of the car, but since the cylinder is narrow it won't amount to much.

The cap needs to be vented because when you press the pedal a small amount of fluid travels down to the slave, then as you release it travels back up again. So air needs to be able to move in and out of the cap.

I don't understand.

[Allan T]

A few weeks after replacing my clutch master cylinder, the new plastic cap cracked. When I'd heard of this happening before, I'd wondered if the caps had been over-tightened, but this one certainly wasn't. I'd not thrown away the at least twenty two year old m/c and cap, so swapped the caps around, and months later the ancient one is good as new.

To have lost so much fluid in so short-a time I suspect yours is cracked, too - and a washer won't fix that.

Jumping the thread when overtightened tends to confirm it.

Good luck!

[Bumpa]

I still don't see how you can lose the majority of the clutch fluid because of a leaking cap. Can you enlighten me? It seems to me that you wouldn't empty the cylinder even if the cap was left off permanently. Why would it rise up and overflow? It makes no sense to me.

If the fluid is emptying around the master cylinder it must be coming out of either the pipe union, or leaking around the piston. Well, that's what I think. Explain what's happening so I can understand.

[Not_Anumber]

Unless Im missing something obvious there doesn't seem any room for doubt on the required action. Bin the cap and replace it.

Driving around with a split reservoir cap with a weak thread that routinely leaks fluid in normal road use is decidedly undesirable and there seems no pressing reason to continue to put up with it.

[AndyC]

Many thanks for the replies. Have attempted to to resolve the problem with a new cap and and old one from previous style reservoir, all to no avail. Still leaks so I think it's going to be a further new reservoir as it seems I have managed to obtain a duff one.

[Peter Cresswell]

I've seen a few questions about problems with parts both on here and also the MGB Register Facebook page. Quite often suppliers cite owners as being unwilling to pay high prices for parts, and so often they offer 2 versions ith quite a large difference in price.
So Andy, I am wondering which Clutch Master Cylinder you purchased - GMC1007 which costs around £95 or GMC1007Z which just under £40? And were you offered each version, or indeed were you aware of the two?
Pete

[Not_Anumber]

I must confess i went even cheaper than that, swapped to a close copy of a Girling that was specified for MGC and various Triumphs. It needed a 'shoulder' removing with a Dremel to fit the pedal box but was then extremely easy to fit and to bleed.

Anyone is welcome to my old Lockheed master cylinder for parts or to rebuild.

[Peter Cresswell]

I am wondering if the leaking cap stems from a mismatch in the clutch hydraulic system you now have. The MGC and the MGB use different master and slave cylinders and by having an MGC master Cylinder and MGB slave cylinder, is the system is forcing fluid back into the master cylinder. The MGB master cylinder is 1.26in bore and the slave cylinder is 0.75in, whereas the MGC uses 0.937in master cylinder and the slave cylinder is 0.625in. This means that when clutch pedal is released the slave cylinder pushes more fluid back to the Master cylinder and might be 'overfilling' it.