Leak from Tokico BM50 Brake Master Cylinder

[brillo]

Hi

My Tokico BM50 Brake master cylinder has a leak from the hole between the two outlets (M10x1) for brake pipes, I have attached a photo.

Can anyone tell me what that hole is and what has failed to allow brake fluid to escape from it, I have looked on line and asked questions on UK forums but can not find the answer, if anyone can enlighten me I would be most grateful.

[SteveL]

On 24/8/2025 at 11:07 PM, brillo said:

Can anyone tell me what that hole is and what has failed to allow brake fluid to escape from it, I have looked on line and asked questions on UK forums but can not find the answer, if anyone can enlighten me I would be most grateful.

I don't get in here much anymore but I can help you with this.   The hole is a vent (air relief) for the brake proportioning valve, which is built into the master cylinder.   

The bad news is that if brake fluid is leaking from that hole then it's getting past the proportioning valve seals.   The really bad news is that no spare parts are available for the proportioning valve either from Nissan or after market.     It's a bit of a PITA getting the proportioning valve out of the master cylinder body anyway but, fortunately, leaks from that area are rare in my experience.

BTW, if those are copper (as such) brake lines you should get rid of them.    Bundy (steel) tube is a far better choice (and legal  in Australia - if that's where you are).

[brillo]

Hi, SteveL

Thank you very much for your reply, you seem to be the only person on the net who has come up with a definitive answer for which I am grateful.

The "Leak" was more by way of wet bubbles when the pedal was depressed hard by a buddy while trying to gey a decent pedal when bleeding the system having fitted the rebuilt BM50 back in the car, which now makes perfect sense.

A bit of a shame having just rebuilt my BM50, I did not touch the proportioning valve side of things, the BM50 was leaking from the primary piston seal and fluid was running down the the Brake booster hence the need to rebuild, I had never noticed any fluid leaking from that hole previously it only started when I refitted it to the car.

The brake lines in the photo are "Kunifer" which is a Copper/Nickel alloy brake pipe, but are only the ones I use to bench bleed Master cylinders, they are perfectly legal to use on vehicles here in the UK, however the lines on the car are PVF coated steel.

Thanks again for clearing this up for me, a purchase of a new BMC appears to be on the cards, I have been looking at various options in case my BM50 was not repairable and have looked at the HFM BM57 which I understand is manufactured in Australia.

 

[GTSBoy]

HFM BM57 has a "bad" knee point, IIRC. It's not the same thing as the later R chassis MC.

[brillo]

Hi GTSBoy,

Excuse my ignorance but what does a "Bad" Knee point mean ? 

[SteveL]

20 hours ago, brillo said:

The "Leak" was more by way of wet bubbles when the pedal was depressed hard by a buddy while trying to gey a decent pedal when bleeding the system having fitted the rebuilt BM50 back in the car, which now makes perfect sense.

The question then becomes - was there any fluid coming from that hole before you did the rebuild ?    You may not have noticed, of course.   Depending on how you did the rebuild, the possibility has to be considered that somehow (cleaning ?)  fluid entered that hole and is now being 'forced'  out by small movements of the proportioning valve.   From the factory, there's actually a small rubber plug in that hole but with age and under-bonnet heat it's quite common for it to 'go missing'.  That rubber plug is designed to allow the venting process but also to prevent 'stuff' getting to the area (prevent corrosion, etc).   The plug is also not available as a spare part AFAIA.

Personally I wouldn't race and buy a new master just yet but keep an eye on the area to see if the 'leak' continues.   If you're concerned about brake fluid damaging nearby paintwork, cable tie a piece of suitable absorbant material over the hole and remove/monitor occasionally.    Operation of the proportioning valve and the brake master itself won't be affected, but also keep an eye on brake fluid level, of course.

In terms of a new master, the genuine part is getting expensive, unfortunately.  Amayama is showing AUD900+.   I was searching recently and there's an aftermarket part available from Japan made by 'Parts Assist':

https://zenmarket.jp/en/yahoo.aspx?q=BNR32%2bBM50&p=1

Whether it's any good I have no idea, although in my experience Japanese aftermarket stuff is pretty good quality-wise.   The above site allows overseas buyers to purchase stuff more easily from Japan but there are fees and shipping costs, of course.   The original Japanese seller page is here:

https://auctions.yahoo.co.jp/jp/auction/p1197401228

Copper/Nickel is fine but I still prefer bundy tube and it's also cheaper.

[niZmO_Man]

I bench bled my HFM BM57 before installing, it leaked badly. Threw it in the bin, because I installed it 2 years after purchase.