Brake threads

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Rob
Posts: 23
Joined: Fri Apr 19, 2013 6:30 am

Brake threads

Post by Rob »

OK ... maybe a silly question. I am using a VW master cylinder.

Is the master cylinder pipe threads 1/8 BPST or 1/8 NPT ?

The thread profiles are 60 and 55 degrees and I don't want any leaks by using the wrong ones.

This may not be a simple answer because I have sourced VW brake cylinder kits from the US that use 3/4" (19.05mm) pistons instead of the European 19.00mm and the piston jams.

I suspect the pipe thread profiles may depend upon exactly where in the world the VW brake cylinders are made.

However, mine has "19" cast on it so I reckon it will use the same threads as whatever VW (Europe) and Steyr Puch used.

Or does everybody just wad the fittings with heaps of PTFE tape and hope for the best ?
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heinkeljb
Posts: 2754
Joined: Sun Dec 30, 2012 12:49 pm
Location: Lewes, East Sussex - UK

Re: Brake threads

Post by heinkeljb »

Hi Rob,

I'm afraid I found your post a little confusing.

I understand you are using a VW master cylinder - These have 4 holes drilled in them to take the brake pipes. One on the end which will have the brake pipe, one on the side which will have the brake light stop switch. Two others which you will have to blank off. Do you have any fittings to go in these holes? If the threads are correct they will screw in all the way to the bottom of the thread. If they are wrong, they should screw in part way and then bind.
At this point, I think you have two choices. A) You source fittings that screw all the way in - i.e. the correct thread. B) You use the fittings you have that bind.

The way the brake pipes are flared is where the seal actually is - the sleeve nuts merely push the back of the flare against the tapper at the bottom of the threaded hole in the cylinder. So as long as the sleeve nut will lock the pipe in tight then you should get a good seal. As an extra precaution against leaks you can wrap the thread with PTFE tape.

From your post, I found it unclear which piston(s) were sticking. The wheel cylinders or the master cylinder? Didn't the wheel cylinders come with pistons already fitted? That is the normal way, as leaking wheel cylinders can be down to scratches on the bores, or worn seals and new cylinders replace both things.

A VW agent should be able to supply the correct brake pipe fittings. Just a question of getting a parts person who is interested enough to go and look in the parts bins and try a few sleeve nuts in your parts to find suitable ones.

John

P.S. where abouts are you? Are you doing a major rebuild or just the brake system?
Haflinger 703AP LWB 1973 - (Once owned by Lady Sutherland & Sons.) Now called "Lurch" !

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Rob
Posts: 23
Joined: Fri Apr 19, 2013 6:30 am

Re: Brake threads

Post by Rob »

Hi John,

I have replaced the Haflinger master cylinder with a VW one of the same 19.00mm bore.

However, it does not have any plugs with it to blank off the extra holes.

I have a plug choice of BSPT or NPT.

I have happened to try out a US sourced VW master cylinder kit from which the piston would not fit since it was made for a 3/4" bore, whereas a European piston is 0.05mm smaller.

So I am now unsure as to what thread profile is correct for the unused threaded fittings. A BSPT plug screws in but I am not sure this is the correct thread angle.
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