5 replies to this topic

[NLinPEN]

This topic relates to my 1996/7 Mini, more specifically to the brake master cylinder (MC) as shown in the photo. Some time ago I refurbished this with a new seal kit GRK1032. All worked fine until suddenly this week it didn't any longer: all brake fluid had leaked out from the reservoir. I then verified that the slave cylinders (rear) and calipers (front) were not leaking, and that all the brake hoses and connections were dry. Which was the case, so the problem was limited to this MC. I topped up the reservoir with fresh brake fluid to see what would happen. The next day all fluid was on the floor, and I could clearly see where it had leaked. The reservoir has two points where it feeds into the MC. Each of them has a black seal, as visible in the photo. The one on the left hand side in the photo (near the ap logo) is leaking; the other one is dry. Also: after removing the MC from the car I could confirm that the MC itself is not leaking: all is dry at the piston, and there is no fluid in the brake booster (servo).

Looking back into my notes I made when I refurbished this MC I notice that I had great difficulty in installing the reservoir on top of the MC. There is/was a fitment issue with the two seals at that location. If I placed the seal over the reservoir tube was I not able to push it into the MC. If I placed the seal in the MC was I not able to push the reservoir on top of it. The seals' wall are slightly too thick to fit properly. So I used a sharp knife to remove a little bit of material to make it fit. It could be that this improvisation is now the root cause of the leakage. I didn't notice it at the time, but now notice that the reservoir can easily be wiggled from left to right. It doesn't seem to be mounted very solidly on the MC.

 

My questions are:

1. Have others had the same fitment problem with these seals between the reservoir and the MC?

2. What solutions are available to have it properly fitting, and still being able to assemble it?

3. Any other recommendations or tips related to this topic?

Attached Files

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Edited by NLinPEN, 16 August 2026 - 11:43 AM.

[Ethel]

Can't speak the quality of the rebuild kit, but  fit the seals to the cylinder then the reservoir will expand the seals for a snug fit. You could use a smear of rubber grease if the reservoir is hard to insert.

 

I'd have been tempted to reuse the original seals, they just sit there.

[Simon@minispares.com]

What brake fluid did you use?

[NLinPEN]

What brake fluid did you use?

DOT4, a bottle with the Bosch brand on it. But to be honest, I am not really picky about the brand of brake fluid I use.

Edited by NLinPEN, 17 August 2026 - 02:31 PM.

[NLinPEN]

I'd have been tempted to reuse the original seals, they just sit there.

In fact, that is indeed an option I am considering. I had kept the seals that I replaced. The previous seals were not damaged, but I did notice that they were less flexible than the new ones. I assumed that more flexible seals would be better than slightly hardened ones. Which is why I took out the old seals and installed new ones (that were included in the repair kit). But it might be that the harder ones are actually a better fit.

Edited by NLinPEN, 17 August 2026 - 02:29 PM.

[NLinPEN]

 

I'd have been tempted to reuse the original seals, they just sit there.

In fact, that is indeed an option I am considering. I had kept the seals that I replaced. The previous seals were not damaged, but I did notice that they were less flexible than the new ones. I assumed that more flexible seals would be better than slightly hardened ones. Which is why I took out the old seals and installed new ones (that were included in the repair kit). But it might be that the harder ones are actually a better fit.

 

In case someone stumbles upon this thread: this is what I ended up doing. I removed the new seals and installed the old seals. This solved the issue: the leakage stopped. Comparing the new seals with the old seals I noticed that the new seals were a bit longer/taller (possibly too tall? I didn't measure to verify). I also noticed that the bottom end of the new seals, the part that sits in the master cylinder, had lost all flexibility and had gone hard as plastic. I wonder whether this lack of flexibility had caused it to leak.

Edited by NLinPEN, 23 August 2026 - 10:55 AM.