11 replies to this topic

[NLinPEN]

I am recently (last few weeks) noticing some symptoms which I can not fully explain. I hope that I can get some help here. It relates to my "Japanese SPI" Mini. Earlier this year (Feb ~ April) I replaced the brake components near all wheels: new hoses, refurbished the calipers, new rotors, new rear slave cylinders and new brake shoes. The brake master (GMC90376) and brake servo (GSM119) appeared to be good so I did not replace these. The vacuum hose with integrated non-return valve looked not so good, so I replaced it with new hose and a separate non-return valve (17H2646). Adjusted the hand brake cables at the rear wheels. Then filled the system with new brake fluid and did a proper bleeding procedure. This resulted in proper brake function.

What I am noticing lately is a difference in behaviour between a "firm step your foot on the pedal" and a "gentle touch on the pedal". With the engine off the effect is the same in both cases: the pedal travel is short and results in firm pressure (can't push any further). When the engine is running I notice a difference in behaviour: when I step firmly on the pedal it behaves the same as when the engine is off: the pedal travels only a short distance and then can not be pushed any further. However, if I push the pedal gently, or use one finger to push the pedal, I can push the pedal down all the way to the floor.

I can't really explain what is causing this effect? The difference between engine running and engine off is the presence/absence of vacuum to the brake servo. But, if there is a vacuum leak I would expect the pedal to, eventually, go to the floor in all cases. Thus also when I push hard on the pedal. This isn't happening. Alternatively, if a seal in the brake master would be leaking I would assume that I would observe the same behaviour both with engine off, and with running engine.

 

What am I overlooking? What could be a possible root cause for this behaviour?

 

Edit: I forgot to mention in my initial post that there is no leakage of brake fluid. The fluid level in the master reservoir is stable since all this time. The connections at all hoses are dry, as are the connections at the master cylinder.

Edited by NLinPEN, 16 June 2025 - 02:44 AM.

[Ratters]

I think I'd go for the master cylinder leaking internally. But also check all new joints connections for external leaks. Finally I don't agree that if you gently and slowly push the brake pedal down that it will go all the way down under any circumstances. Good luck.

[Spider]

It does sound like Master Cylinder to me. While you're at it, since everything else is new, may as well replace the Shut Off Valve on the bulkhead too.

[NLinPEN]

It does sound like Master Cylinder to me. While you're at it, since everything else is new, may as well replace the Shut Off Valve on the bulkhead too.

I'm sorry for my ignorance: what do you mean with the shut off valve? I didn't know that the brake system has a shut off valve?

[Spider]

I'm sorry for my ignorance: what do you mean with the shut off valve? I didn't know that the brake system has a shut off valve?

 

 

This one;-

https://www.minispar...d-in-engine-bay

 

And on your car, it would normally be around here;-

 

 

 

[NLinPEN]

Thank you Spider! I wasn't even aware that my car has such a limiter. I will verify its actual location in my car.

Is there a specific reason why you recommend me to replace it? Are these prone to fail?

 

The more I think about it, the more I start to see that there might be a seal leak in the master cylinder. And how the presence of vacuum in the servo may emphasize the presence of the leak. I guess it is time to take out the master cylinder and replace all seals. I have done this job previously on another car (not Mini), so this isn't my first rodeo.

[bpirie1000]

Try taking the brake line off the master cylinder and putting the bleed nipple straight into master cylinder. Smallest of bleeds then press pedal. If master is leaking then you will have a soft pedal... eradicating any other joint failure.

[Spider]

Is there a specific reason why you recommend me to replace it? Are these prone to fail?.

 

Well, you have replaced all the other major hydraulic components and these do fail, so to me, it does just make sense.

 

As an FYI, while I would think only few would do so, the factory does recommend that all rubber parts of the Brake System be replaced every 3 years or 40,000 miles (which ever is sooner). I do my own about every 5 years.

[NLinPEN]

Today I made a start with removing the brake master cylinder. I want to inspect the seals and, if necessary, replace them. Unfortunately, rather soon I stumbled upon a roadblock and haven't found a way forward yet. I removed the circlip and was then able to remove a washer and a first cup-shaped seal. Then I could see a white plastic/nylon part in the cylinder. But I can not get it out, and thus can't get the pistons and seals out.

What is the trick to remove this white part (and whatever is coming behind it)?

 

One other thing I noticed is that the surface of the brake servo where the master cylinder is mounted on is rather rusty and pitted. I have the impression that it used to be painted black just like the rest of the servo, but that this paint got damaged by leaking brake fluid in the past (currently it is dry: no brake fluid leakage). There is a rubber O-ring to seal it off. But it could be that this O-ring is not good enough. I will clean up this surface and put some new paint on it. The O-ring is in good condition but I plan on mounting an O-ring with a slightly thicker cross-section to create a stronger seal.

[Spider]

There doesn't appear to be anything 'trick' holding that in place;-

 

 

 

It might need a good bang on some timber to get it moving.

Note that the Secondary Piston however, does have a hidden pin (9) holding that in.

[NLinPEN]

Thank you Spider for posting this exploded view drawing. This was very helpful for me to understand what to expect. And to learn that I needed to remove pin #9 to get the second piston and spring out. I had not been able to find that kind of drawing online. Anyway, after using plenty of WD40 and sufficient "convincing" I had it come apart. It was indeed the white nylon piece that was stuck in the front chamber (not the bore of the cylinder). Part #18 on the right hand side of the drawing consists of five pieces. The white nylon ring that was stuck is the middle one of the five.

The seals in the cylinder look good and might be not that old yet. But I do have a new seal set (GRK1032) and will replace the seals. The bore looks perfectly clean and shiny, there is no pitting or other irregularity. The "front chamber" does have some pitting, probably caused by previous brake fluid leakage. I have cleaned that up a bit and am now going to put everything back together.

[NLinPEN]

The job is completed. After disassembling the master cylinder I could see all its components. The bore was in good condition: no pitting, corrosion, or any other damage.

 

 

 

Then started the assembly phase, with all new seals. Five seals go in the bore, two are between the cylinder and the fluid reservoir. I used a "homemade press" to compress the spring in the bore while at the same time fiddling to get the circlip in place.

 

 

I faced two problems during assembly: the white nylon/plastic ring is too wide to fit (diameter too large). I carefully sanded it off until it had a snug fit. The other issue was the seals that go between the cylinder and the fluid reservoir. After a long struggle I concluded that the seals' side wall is too thick. After removing some from the outside of the sidewall I was able to mount the fluid reservoir.

The seal kit came with a new O-ring for the interface between the master cylinder and the brake booster. This ring's cross section is a bit thin and I think that it does not provide sufficient sealing. So I bought locally a slightly thicker ring (4 mm instead of 3 mm) and mounted that instead. After installing everything and topping up with brake fluid I waited one day for air bubbles to come out. Then, although I didn't expect any bubbles, I did a brake bleeding for good measure.

 

This seems to have solved the issue: the brake pedal is hard, and pedal travel is short. Now, when lightly touching the brake pedal am I no longer able to push the pedal to the floor. Although I did not find a damaged seal, or anything else damaged, replacing the seals has solved the issue. There must have been some sort of internal leakage in the master cylinder.

 

One issue solved. But as it is with old cars: up to solving the next issue.

Attached Files

•  explodedview.jpeg   63.02K   0 downloads
•  assembly.jpeg   36.68K   0 downloads

Edited by NLinPEN, 26 June 2025 - 12:16 PM.