Leaky brake reservoir cap

[KnifeySpoony]

I have a recent 420R (LHD) with the AP Racing "race" cap with the bellows and no sensor- installed from new, as recommended to reduce/eliminate leaking from stock cap. It didn't leak a drop until last trackday it was leaking something fierce. Not sure what changed. I have a towel zip-tied over the cap as a precautionary measure to absorb any seepage. After each session on track Thursday, the towel was fully saturated and there was mist/spray around the area under the bonnet (presumably flinging off the wet towel). Changing out for a new towel each session would just result in another saturated towel. Made sure it was quite tight (can it be too tight?). Made sure the bellows was well-seated into the cap before screwing it on. Tried putting the bellows down into the reservoir, and then applied the cap. Nothing helped. I know some people on some cars recommend running much less fluid in the reservoir (say, only 50% full) to minimize leaks. I have always kept it topped up to "full", however over the course of the day, it ran down to about 75% full with no less leaking.



Just now I cut a small ring of paperboard (cereal box lol) to fit into the ring groove in the cap to give just a little more downward pressure on the bellows flange to see if it helps seal more. Maybe something thicker would be of more help. Won't know until next time out on track next Saturday. It doesn't leak with street driving.



Anyone dealt with this issue? Any hot tips?

[Blue 7]

When I bought my 2017 420R three years ago, I had a slight leak from the standard cap.

It seemed to me that the cap was bottoming out before the rubber seal was compressed, there was no sense of rubber ‘squish’ as the cap tightened down, it would just come to an abrupt stop.

So I added a second rubber seal underneath the cap & that sorted it. I haven’t had any leaks since, even on track. I run the car with the reservoir quite full, to avoid the ‘low brake fluid’ light flickering, but I still haven’t had any problems.

[AndrewGP]

Blue 7 said:

When I bought my 2017 420R three years ago, I had a slight leak from the standard cap.

It seemed to me that the cap was bottoming out before the rubber seal was compressed, there was no sense of rubber ‘squish’ as the cap tightened down, it would just come to an abrupt stop.

So I added a second rubber seal underneath the cap & that sorted it. I haven’t had any leaks since, even on track. I run the car with the reservoir quite full, to avoid the ‘low brake fluid’ light flickering, but I still haven’t had any problems.

That’s a great tip, thank you! Mine leaks a little too, nothing major but annoying nonetheless. I cut the top of an old sock (the straight tube bit above the ankle) and wrapped that round the cap to soak up any excess fluid and it works well. But your solution sounds much better

[Boggo]

Sometimes you can overtighten and the rubber seal actually slides down inside the res - have a check for that happening - otherwise I would look at replacing the cap, not expensive.

Also, use some cheap sweat bands over the cap to absorb any spills/leaks - absorbs alot more than paper towel.

[KnifeySpoony]

Boggo said:

Sometimes you can overtighten and the rubber seal actually slides down inside the res - have a check for that happening - otherwise I would look at replacing the cap, not expensive.

Also, use some cheap sweat bands over the cap to absorb any spills/leaks - absorbs alot more than paper towel.

I do think it's possible I overtightened.

I am using a cloth dish towel. I have heard of some using a sock - that would be even more absorbent and I may resort to that.

[Steve Campbell]

Wrist sweat bands is the place to look. Elasticated and fit really well.

[KnifeySpoony]

What I ended up doing was add a ring of thin cardboard in the cap to add additional pressure on the bellows, then took car to not overtighten. Held up through an entire trackday with no significant leakage. Fingers crossed it will stay that way.